Firefox Headlines (July 22nd, 2021)

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Doug Thayer

Jul 22, 2021, 11:59:05 AMJul 22
to firefox-dev
Hello, Firefox fans!

It's here again - your fortnightly Nightly update. Every(ish) two weeks, a bunch of Firefox engineers get together and provide updates on what we're working on. A few of us stay after the meeting and collect the notes to send out to you fine folks! So without further ado, here it is:


  • Happy Firefox 90 Day!

  • tkikuchi added a new about:third-party page to visualize third-party modules injected into Firefox processes (Windows-only)

  • Ava made it possible to queue a download to be opened when it’s finished by clicking on it from the downloads panel (Bug 1711053). Available behind the pref:

  • Simplify Page for Printing has been reintroduced in Firefox 90

  • In 91, the Fission Beta experiment is aiming to enable Fission for 15% of the population, including Linux users.

  • The Nightly rollout is being increased to 70%. At this point, more than half of our Nightly population has Fission enabled!

    • You can check to see if you have Fission enabled by going to about:support, and checking the “Fission Windows” field. If it shows a number greater than 0, congratulations! You’re running with Fission.

  • Improved dark mode support on macOS will launch in Firefox 91! (bug 1623686)

Friends of the Firefox team

For contributions from June 29th to July 13th  2021, inclusive.


Resolved bugs (excluding employees)

Fixed more than one bug

  • Ava Katushka [:ava8katushka]

  • Kajal Sah [:kajalsah107]

  • Matthew N. [:MattN] ❤️

  • Michelle Goossens [:michelle]

New contributors (🌟 = first patch)

Project Updates

Add-ons / Web Extensions

Addon Manager & about:addons
  • The theme previews for the builtin themes listed in about:addons have been updated to match the new Proton UI - Bug 1708717

WebExtension APIs
  • As part of the work related to the Manifest Version 3 WebExtensions, we Introduced a new “action” WebExtensions API namespace (in practice it is an alias of the current browserAction API, restricted to extensions with manifest_version set to 3) - Bug 1706398

  • Some new WebRender-related browserSettings have been introduced in Firefox 91:

    • browser.browserSettings.colorManagement.mode (values: “off” / “full” / “tagged_only”) and browser.browserSettings.colorManagement.useNativeSRGB (values: true / false) - Bug 1714428 

    • browser.browserSettings.colorManagement.useWebRenderCompositor (values: true / false) - Bug 1719688

Downloads Panel

  • Other new changes behind the pref:

    • Make “Save to disk” the default decision for files (Bug 1710941)

    • Download panel will automatically open when a download starts and there are no other downloads in progress (Bug 1709129)


  • In 91, the Fission Beta experiment is aiming to enable Fission for 15% of the population, including Linux users

  • The Nightly rollout is being increased to 70%. At this point, more than half of our Nightly population has Fission enabled!

Lint, Docs and Workflow

  • glandium turned on –enable-bootstrap by default

    • This means toolchains and other build dependencies (like node) will automatically be updated on `mach build` without the need to run `mach bootstrap`

  • marco turned on warnings in reviewbot. These are optional fixes which won’t cause a back out. This is the first time they are being surfaced by default

    • ESLint severity 1 issues are warnings

    • Other optional linters (like codespell) are also warnings

  • See full Engineering Effectiveness Newsletter for more

  • Standard8 landed a change to improve the documentation for our Mozilla specific ESLint rules.

macOS Spotlight

  • spohl is working on a fix for misaligned stoplight buttons in RTL locales (bug 1419375)

  • Work continues on improved fullscreen support, including being able to exit fullscreen with the Esc key (bug 1688584)

  • Work continues on improving memory pressure detection, which will reduce the number of prompts asking that apps be force quit.


Performance Tools


  • pbz updated the site data manager in about:preferences to group storage by base domain (eTLD+1) and support clearing storage per “cookie jar”. Bug 1711869, Bug 1712170

  • vringar is working on making the site data clearing confirmation dialog nicer when we only clear data for a single base domain. Bug 1715499

  • vringar is adding a confirmation dialog before clearing data via “Forget About This Site”. Bug 1711759


Search and Navigation

  • Harry fixed support for $ (url) and # (title) restriction tokens in the urlbar, though note they cannot be combined anymore with other restrictions. Bug 1719180 

  • Harry converted the last remaining old style urlbar provider to a modern one - Bug 1717509 

  • Gijs changed fixup to not generate alternate www.[...].com addresses for “http” and “https” strings. Bug 1719442 


  • Kajal has gotten started on porting over screenshots to its own browser component.. This work is behind the `screenshots.browser.component.enabled` pref and we expect to land the initial parts in the coming weeks.

Below the fold

  • [mconley] Joy of Profiling - Open to the public!

    • Mondays, 10AM ET (Zoom ID: 98769155293)

  • [mconley] Recent kerfuffle in Mercurial-land with reload.svg

  • [Standard8] A reminder to use the phab-test-policy add-on to remind you to add the testing policy when approving a patch in phabricator.

  • [Standard8] Also a reminder to investigate the phab-conventional-comments add-on to help you make it clearer what each comment in a review is (e.g. nit, suggestion, issue etc).

  • [mconley] Using HTML custom elements in browser UI? Great! You can inherit from MozHTMLElement to get inheritedAttributes support. Just don’t forget to call initializeAttributeInheritance after binding to the DOM.

  • [vringar] You can put add_task(<task definition>).only(); on a mochitest to only have this test run

    • You can also use .skip() to skip tests you aren’t interested in

This information, including all of the previous updates, are fully available in this doc:

Also note that I'm really just synthesizing and humanizing the stuff that's on the Wiki, which is freely available from here:, so check that out if you want the source material. It'll also be posted (minus the Below the Fold stuff) to the Nightly blog.

Finally, if there's a Firefox-ish team that you feel isn't being represented here, please let them know about the above meeting so we can get their updates.

Keep on rockin' the free web,


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