Categories: News

These Weeks in Firefox: Issue 51


Friends of the Firefox team


  • New Student Project: Fluent Migrations (watch out for email)

Resolved bugs (excluding employees)

Project Updates

Add-ons / Web Extensions

  • Ongoing work on:
    • User opt-in for extensions in private browsing windows
    • Rebuilding about:addons in HTML
  • Old Lightweight Themes (LWTs) on will be converted to XPI packaged themes next week

Browser Architecture

Developer Tools

Huge shoutout to debugger community for all of their work during the winter break!

Layout Tools
Remote Debugging
Fission Support


  • New student project started up last week. Goals are:
    • convert more strings to Fluent
    • increase tool support
    • research porting the fluent-rs parser to wasm and replacing the JS Fluent parser with the wasm fluent-rs parser



  • In Q1 we are focusing our efforts on startup performance. This time we’ll care both about first paint performance (which already received optimization efforts previously and is close to parity with Chrome) and the time to render the home page.
  • Doug landed his document splitting work that should enable faster rendering and is investigating creating a warmup service to preload Firefox files when the OS starts.
  • Felipe’s tab animation patch is going through review.
  • The code of the old about:performance is gone (both front-end and back-end pieces) and some test coverage was added.
  • Gijs continued his browser adjustment work (adding telemetry and about:support visibility), improved Pocket’s startup behavior, and removed some more Feed code.
  • mconley is unblocking enabling the background process priority manager, by removing the HAL stuff that was leftover from FxOS.
  • Perf.html improvements deployed recently:
    • Tooltips in the thread activity graph indicating the meaning of colors (most frequent request we got during the all-hands!)
      • A tooltip is floating over the colour-coded visualization of processing time in perf.html. The tooltip describes what the hovered colour means.

        This was a top request from users of the Profiler!

    • Memory track
      • A new track in the perf.html profiler view shows a graph of memory usage during the recorded time.

        This should help us notice memory allocation patterns in profiles.

    • Category colors in the stack chart
      • The stack chart is now colour coded, using the same colouring system as the thread tracks.

        This should help people narrow down slow code to the responsible components.


  • We’re eliminating some interesting performance regressions from enabling cookie restrictions by default.
  • Erica is working on another Shield study on content blocking breakage
  • Baku refactored some of URL-Classifier to prepare it for future endeavors, including a neat new way to manually classify URLs on about:url-classifier

Search and Navigation

Quantum Bar

3 comments on “These Weeks in Firefox: Issue 51”

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  1. Jan wrote on

    Work continues on switching built-in search engines to WebExtensions, intention to land after the next merge

    Is this only internal refactoring or will the OpenSearch support be removed? I have 20-something search modules and converting them to web-exts and uploading them to AMO would not very convenient.


  2. Gabriel wrote on

    When pop up video in firefox ?


  3. Thomas wrote on

    Is there any plan to move tabs from a bloated process to a new process, before killing the bloated process, to prevent Firefox from eating all the memory and getting slow when trying to free some memory?
    While it uses more memory in the beginning, Chrome’s one process per tab avoids that kind of problem and keeps it fast.
    Firefox’s multiple tabs per process is nice to save memory but it requires a way to move tabs to a new fresh process from time to time.


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