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Actions from last meeting

  • Select a new meeting leader for May 23rd meeting.
  • Today’s meeting leader is: Tim Spurway
  • Next meeting leader is: mconley

General Topics / Roundtable

(tips/tricks/FYI, productivity improvements, industry news)

  • [mconley] ← a tool from Facebook that does JS optimizations, with an emphasis on runtime performance. Might be worth keeping an eye on this.

Friends of the Firefox team

(Give a shoutout/thanks to people for helping fix and test bugs. Introductions)

Project Updates


Activity Stream

  • Get Activity Stream (Dev channel)
  • Pocket in Activity Stream experiment launched (Test Pilot version)
  • Activity Stream is landing in nightly and is available behind pref (browser.newtabpage.activity-stream.enabled)

Electrolysis (e10s)

  • gabor turned on the pre-allocated process manager, which should improve the perceived performance of opening tabs and windows in new processes
  • mconley is currently fixing a regression in the tab switch spinner metric
  • e10s-multi A/B test is currently underway on Beta
  • e10s-a11y support still targeted for Firefox 55

Firefox Core Engineering

  • Shield Study defaulting to click-to-play (Plugin Safety) has begun on Release 53 and will run through June 15.

Form Autofill

Go Faster

  • There’s an example add-on repository that is intended to provide example integrations for legacy system-addons based in github. This includes taskcluster integration for unit tests. More testing functionality, e.g. mochitests, probably coming soon.
    • This was originally based on ideas from Hello.
  • There is also an example web-extension repository that was based on that idea, but is intended as a more general add-on developer repo that can be used outside the Mozilla circle, e.g. doesn’t use taskcluster. More info here.




  • [mconley] Published Performance best practices for Firefox front-end engineers 🔥
  • We are working on deterministic perf tests for sync reflows and files loaded too early during startup
    • Sync layout and style flush tests will be in > browser/base/content/test/performance. When these become > available, make sure to run these when you add Photon-y > things!
  • Expect big patches to land to stop using Task.jsm in browser/ and toolkit/, and stop using the non standard Promise.defer from promise.jsm
    • If you have WIP patches for these folders, consider landing them > soon to avoid bitrot.


  • No updates.




  • [Fischer] We checked out the onboarding overlay prototype with the team members. And have a discussion with Activity-Stream team on how to integrate our onboarding overlay into Activity-Stream as well as how to communicate between the tour notification bar and the snippet using the pref. Then we are targeting to sort out how to approach the bug 1054947, bug 1361286 and more about the onboarding overlay as a System addon.
  • [Rex]
    • Scenario for onboarding overlay after sync account login: we > decided the behavior after user successfully logged in.
    • Prototype for making onboarding overlay as a system add-on and > find out issues that need to be discussed further.
  • [Fred] We checked out the automigration UX spec with UX and also the activity stream engineer.
    • Figured out how to communicate with AutoMigrate moduels by > messaging.
    • The actual implementation place is in discussion.
    • Prototype based on the newtab page is on the way.


  • [timdream] Photon work week! We’ll have the scope of the Photon Preferences project finalized this week. The scope includes follow-up and ship preferences re-org, search, and updates visual design and content (graphics and wording). Check out and please file bugs under meta bugs.

Platform Audibles

  • No updates.


  • jkt is looking into rewriting containers to use WebExtensions instead of the Addon SDK
  • Our three Outreachy interns for May 30 - August 30 were announced last week:
    • Prathiksha G - Site Permission Management UI 🎉
    • Princiya Marina Sequeira & Bianca Danforth - Upgrade Lightbeam 🎉

Project Mortar (PDFium)


Sync / Firefox Accounts

Test Pilot

This week I learned

  • [kit] `nsIConverterInputStream` reads 8192 bytes at a time, even if you pass a larger count to `readString`. Make sure to read the decoded output in a loop:
  • [mconley] We should strive to batch all DOM writes into requestAnimationFrame where possible. This helps us to avoid sync style / layout flushes in event handlers.