Firefox Proton Theme Tweaks


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Firefox 89 was just released with the new Proton UI. While a new UI was extremely low on my list of desired Firefox improvements I have to admit that for the large part it looks nice. I am especially a fan of the new menu, which I find much easier to navigate on the rare chance that I need to use it.

However, I did find that there was a lot of unnecessary space around the tabs, so I was wondering if I could cut that down. I think I have manged to cut down the space in a way that actually makes the browser look cleaner overall, so I figured I would share.

For reference this is how my Firefox looked with no changes.

Firefox with stock theme


These changes will use the userChrome.css file to add custom styles to the browser. While this voids your warranty it is very easy to revert (just delete the file) so it doesn’t hurt to try it out.

First go to about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true. Then create a file called chrome/userChrome.css in your profile folder. You can find you profile folder by going to about:profiles. If you need more detailed instructions has an in-depth guide.

Full-height Tabs

I think this looks really nice. Instead of tabs being “buttons” they just stretch the full height from the URL bar to the top of the window.

Firefox with full-height tabs

To get this effect simply add the following to your userChrome.css file and restart your browser.

.tab-background {
	--tab-block-margin: 0;
	--tab-border-radius: 0;

Pinned Tab Gap

By default, there is a noticeable gap between the last pinned tab and the first regular tab. I thought this looked awkward, so I just removed it. I think the result looks quite nice.

Firefox with no gap after the pinned tabs

Just add this to the userChrome.css file.

#tabbrowser-tabs[haspinnedtabs]:not([positionpinnedtabs]) > #tabbrowser-arrowscrollbox > .tabbrowser-tab[first-visible-unpinned-tab] {
	margin-inline-start: 0 !important;

Other Changes

These are the small tweaks that I chose, but you can easily change anything else in the UI thanks to Firefox’s use of HTML and CSS. The easiest way to do this is open the Browser Toolbox. You can then use the “Inspector” tab to identify elements that you want to change and edit the CSS. Once you find something that you like you can add it to your userChrome.css to make it persistent.


Here are the images above each other for easy comparison.

Firefox with stock theme

Firefox with full-height tabs

Firefox with no gap after the pinned tabs

And a comparison of the vertical space savings. Not explicitly a goal but a nice bonus.

Comparison of vertical space usage