fidzu : thunderbird

Hi Mozillian,

In this edition, we're exploring the new Firefox design, Mozilla's recent partnership with Mistral to expand choice in AI-powered browsing, and several ways to get involved with MozFest. You can join a livestream about the value of bringing open-source communities together in person, volunteer at MozFest in Barcelona, or take part in the latest privacy discussion from the Firefox community on Reddit.

Read on for the latest updates and opportunities to participate!

More modern. More flexible. Still Firefox.

Firefox 157 introduces a new Firefox design with updated colors, icons and themes that make the browser feel more modern while preserving the familiar Firefox experience. The release also brings back Compact Mode and adds easier theme selection, new wallpapers and more ways to customize the New Tab page. Underneath the new look, Firefox remains independent and open source, with built-in privacy protections and controls that let you decide how your browser works, including how AI features appear.

Learn more

Mozilla and Mistral partner to expand AI choice

Mozilla and Mistral have announced a partnership aimed at increasing competition and preserving user choice in AI-powered browsing. As part of the collaboration, the open-source Mistral Small 4 model is coming to Firefox Smart Window Beta as a new option for users in the US and Canada. Smart Window Beta is also expanding to France with official French-language support, with additional European markets planned. Users can continue choosing from multiple AI models, reinforcing Firefox's commitment to avoiding lock-in and keeping the web open to different technologies and providers.

Learn more

Does open source need IRL? The case for MozFest

Is showing up in person important for open source? Join the Owners Not Renters livestream on Tuesday, September 29, to hear the story behind MozFest and explore why gathering in person still matters for open-source communities ahead of MozFest, taking place October 28-30 in Barcelona. Kali Villarosa (MoFo), Seher Shafiq (MoFo) and Brian Behlendorf (co-founded Apache) will join host Marcus Rein for an open conversation, followed by a live Q&A. Bring your questions and your hot takes!

Watch and participate

MozFest call for Volunteers

From 28-30 October 2026, the Mozilla Foundation will host the Mozilla Festival in Barcelona, bringing together people from diverse fields to explore how we can work collectively to "re-wild" the web. To help bring the Festival to life, we're building a team of volunteers. By joining, you'll help deliver a major event in support of Mozilla's mission while also having opportunities to participate in sessions throughout the three-day Festival.

Sign up to volunteer

From the Reddit Community

Privacy conversations are going mainstream, and Firefox has been at this for 20+ years. In light of a recent video making the rounds about the importance of privacy, the Firefox Team shared a rundown of built-in privacy features (ad blocking, tracking protection, encrypted sync, a free VPN, and more). Jump in at r/firefox and share your own privacy tips and configs!

Join the discussion

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P.S.

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30 Sep 2026 5:01pm GMT

On September 29, Firefox 157 shipped Nova, the biggest refresh of the Firefox interface since Proton back in 2021. The browser frame is cleaner, the sidebar plays a bigger role, and vertical tabs are a first-class layout. Your theme continues to load, and most color and image properties work as they did before.

What does change is where those colors and images display, how some of them look next to each other, and a handful of properties that no longer have a visible effect. Most themes will look fine. Some, especially themes built around a background image, look different enough that you will want to take a look.

This post walks through what changed compared with the Proton interface, what to check in your theme, and what is still being worked on. If you only have five minutes, jump to the checklist.

What changed in the browser frame

The toolbar and sidebar now sit flush with the window edges, with no gaps between them.

For your theme, this means your frame color and background image have less empty space around them, and the edges where the toolbar, sidebar, and page meet are more visible.

Area	Proton	Nova (Firefox 157)
Toolbar and sidebar	Flush with the window, no gaps or floating borders	Same as before
Horizontal tabs	Theme background applied to the tab strip	Same as before
Vertical tabs	Tab bar color on the tab strip	Tab bar color also shows on the toolbar
Sidebar	Separate panel	Shares the theme background with the toolbar area

colors, accents and tabs

Accent color. The default Firefox theme uses Nova's purple accent. Every other theme, including yours, uses the operating system's accent color for buttons and other accented controls. Themes cannot set the accent color through the manifest, and is why your theme cannot show the purple you see in the default theme.

Focus and link colors. Firefox still applies its own blue and cyan to focus rings and some links when a theme other than the default is active.

The selected tab. Proton always drew a shadow under the selected tab, which helped it stand out even when a theme did not style it. Nova removes that shadow. If your theme relies on it, the selected tab can blend into the tab strip. Set the selected tab colors, with tab_selected and tab_line, so the selected tab stands out.

Button hover colors. Your button_background_hover and button_background_active colors also apply to buttons on the tab strip, and for buttons that sit on your frame background with vertical tabs. A solid color picked to match the toolbar can almost disappear there. Semi-transparent colors, for example rgba(0, 0, 0, 0.15) for hover and rgba(0, 0, 0, 0.3) for pressed, work on both backgrounds.

Sidebar and vertical tabs

• The sidebar shares your background. The theme background now continues under the sidebar, so the sidebar and the toolbar area appear as one surface.
• Set sidebar colors explicitly. Use sidebar and sidebar_text for sidebar panels. If you leave them out, Firefox picks colors for you, relevant to your theme's color.
• sidebar_border is now a separator. The sidebar splitter is gone; the property now colors the line between the sidebar and the page.
• Sidebar buttons follow your text color. Buttons and icons in the sidebar are derived from your text color rather than your toolbar button colors, so they can look slightly different from the toolbar.
• Vertical tabs and the toolbar. With vertical tabs, your tab bar color is also applied to the toolbar. If those two colors were chosen to contrast, check how they look together.
• Expand on hover. In Firefox 157, a sidebar that expands on hover may not show your theme's background. This is fixed in Firefox 158.

Properties that changed or no longer work

Some theme properties behave differently in Nova. Your theme still loads if it uses them; they just have no visible effect, or style something different.

Property	In Nova
ntp_text	No longer styles the new tab page search box
popup_text	No longer styles the address bar dropdown; other popups still use it
sidebar_highlight	No visible effect; the default highlight color is used
sidebar_highlight_text	No visible effect
toolbar_bottom_separator	Acts as a separator next to or below the toolbar and around the page

The theme manifest reference on MDN is being updated with these changes.

Themes with background images

Themes using images are where you are most likely to see a difference. Your image can now show behind the sidebar and vertical tabs, and an image designed for a wide, short toolbar rarely looks good stretched or tiled down a vertical panel. In our testing of popular themes, the image themes that look off in the sidebar mostly did before Nova too. We did test a number of themes but we invite you to do your own testing. Nova makes the sidebar more visible, so the issues are easier to notice.

You can now choose where your images go with the backgrounds_area property, available from Firefox 156:

• "auto" (the default): Firefox decides from additional_backgrounds_alignment. If any image is aligned to the vertical center or bottom, images stay in the top toolbars; otherwise they cover the whole window.
• "window": images cover the whole window, including behind the sidebar and vertical tabs.
• "top_toolbars": images stay in the toolbars at the top of the window, and the sidebar uses your frame color.

If your image was made for the toolbar only, "top_toolbars" with a frame color that suits the image is usually the quickest fix.

Also set a toolbar color, even a semi-transparent one. Without it, notification bars that appear above the page sit directly on your image and can be hard to read.

New Gradient Support

Nova's color themes use gradients, and you can too. The Nova theme manifests are the best working example; look at how they set colors for each surface, including the sidebar. backgrounds_area applies to gradients the same way it applies to images. Older versions of Firefox ignore color properties they don't recognize, so your theme still loads there; if you use a feature older versions reject, set strict_min_version in your manifest.

Checklist: what to check in your theme

Install your theme in Firefox 157 and look at it in each of these states:

1. Horizontal tabs and vertical tabs.
2. Sidebar closed, open, and expanded on hover. Also toggle "Customize sidebar" while the sidebar is open or expanded.
3. Compact density, if your users are likely to use it (under settings -> appearance density(.
4. A private window.
5. Light and dark system settings, since the accent follows the operating system.
6. Hover over the buttons on the tab strip.

Then, if something looks off:

• Set tab_selected and tab_line so the selected tab stands out without Firefox's old shadow.
• Set sidebar and sidebar_text to colors that match your theme.
• Use semi-transparent button_background_hover and button_background_active colors.
• Check that your tab bar and toolbar colors work together with vertical tabs enabled.
• For image themes, choose a backgrounds_area and set a toolbar color.
• Remove or ignore properties that no longer have an effect.
• Publish an update on AMO so your users get the changes.

What we are still working on

Nova will evolve after launch. Firefox 158 ships on October 13 with the expand-on-hover sidebar fix. We are also working on these, with no release date yet:

• Focus and link colors that fit your theme instead of the fixed blue and cyan.
• A popup_icons color for icons in menus and panels, separate from toolbar icons.
• Theme colors for hover and selected states inside sidebar panels.
• Better hover states for tab strip buttons.

We will post again when these land.

For users who prefer the old look

Some of your users may miss Proton. From September 29, Mozilla is featuring a Firefox Proton theme in the Add-ons Manager for anyone who wants the previous look. It is a normal theme, so users can switch between it and yours at any time.

Resources

• Theme manifest reference on MDN: every color, image, and property, including backgrounds_area.
• Static themes on Extension Workshop : updated for Nova.
• Nova theme manifests: worked examples for gradients and sidebar colors; the themes themselves are on AMO, for example Nova Sun.

Questions?

We want your theme to look its best in Nova. If something looks wrong and the checklist does not fix it, tell us on Discourse and include a link to your theme and a screenshot. Those reports help us find the cases that need a Firefox fix rather than a theme change.

Thank you for making Firefox look like you and like your users.

The post Nova is here: what changes for your Firefox theme appeared first on Mozilla Add-ons Community Blog.

29 Sep 2026 3:34pm GMT

What comes to mind when you think of Firefox? Is it our best-in-class privacy features and ad blocking capabilities? Our partnerships with Wrexham, NVIDIA or Mistral? Or maybe our adorable mascot Kit?

If you've been following Firefox this year, you've seen us add new ways to get things done while giving you more choice over how you browse. That includes browsing two pages side by side, adding an extra layer of privacy with built-in VPN, blocking trackers for smoother scrolling and faster load times, and deciding how AI shows up in Firefox.

For the past few months, many of you have also been trying out our new Firefox design in Nightly and sharing your thoughts with us as we continued to refine the experience.

Today, we're rolling out that design with FX 157 to everyone using Firefox across desktop and mobile devices. Firefox has done a lot of growing over the last year; now it looks the part.

A more modern Firefox

When we first shared our plans to design Firefox for the future, we said we wanted Firefox to feel "current, but not generic. Warm, but still precise."

That design ethos now carries across all of Firefox. We've refreshed the colors, icons and themes throughout the browser, from the tabs and toolbars you use every day to your New Tab page, Private Browsing experience and more, with no cost to your browser performance.

This shared design language across desktop and mobile gives Firefox a consistent foundation for new features and experiences, while keeping the personality that makes Firefox unique.

Make Firefox yours

We've added more choices for how Firefox looks and works, including the return of a feature many of you asked for:

Compact Mode is back. As people started trying the new design, we heard from many of you who wanted to fit more into your browser window. Compact Mode reduces the size of your tabs and toolbar to give more of your screen to the web, with an auto-compact option for smaller screens.

Find your look. A new theme picker makes it easier to explore different looks for Firefox, with new themes and wallpapers to choose from.

Set up New Tab your way. Choose what you want to see on your homepage. Now, you have a dedicated space to pin and organize shortcuts so sites you use all the time stay put.

Refined, not reinvented

Themes and wallpapers may change over time, but the things that made people choose Firefox in the first place are still the same. We're still independent and open source, with privacy protections built into the browser and controls that put you in charge of how Firefox works for you.

This carries through everything we build, from the height of your toolbars and what belongs on your New Tab page to which AI features show up in your browser, which model you choose and what context you want it to have.

Today's Firefox is a renewal, grounded in the same principles that have shaped it from the beginning, with a new design built to carry Firefox forward.

For those who have come with us on this journey, thank you for helping us shape the future of Firefox.

If you've been away on hiatus, welcome back. Try the latest Firefox and let us know what you think.

The post More modern. More flexible. Still Firefox. appeared first on The Mozilla Blog.

29 Sep 2026 1:00pm GMT