Project Fission

Fission is Mozilla's project to implement Firefox site isolation: third-party iframes will be loaded in separate processes, safely sandboxed from the web page's content process.

Contact

The Fission team is standing by, ready to answer your questions in the #fission:mozilla.org room on Mozilla's Matrix server.

Reporting Bugs

To file a Fission bug in Bugzilla, click here to use this Fission bug template. Or file a bug and include the word “Fission” in the bug summary. The Fission team’s bug triage will find the bug, regardless of which Bugzilla component you file it in.

• Fission meta bug 1451850
• All bugs blocking Fission Nightly

Known Issues

• Some extensions might not work fully. But please file bugs for any of these issues!
• Some video player issues on YouTube and Twitch. (bug 1619370, bug 1620341)
• Cross-site iframes appear empty when printed.
• Cross-site iframes appear empty in screenshots.
• Documents with cross-site iframes cannot enter the BFCache.
• Session history is not tracked for cross-site iframes.
• Session restore occasionally fails to restore page state, such as page zoom, scroll offset, or form data.
• DevTools support for cross-site iframes with fission is incomplete.
• Attempting to debug extensions or workers from a non-fission window can cause crashes.
• Resource exhaustion issues with large tab counts, including:
  • Excessive memory usage
  • File descriptor exhaustion issues
  • X11 connection exhaustion on Linux
• Picture-in-Picture doesn't work on cross-origin iframes (bug 1575868)

Enabling Fission

A "[F]" in a loaded tab's tooltip indicates that Fission is enabled

Fission is still in active development, and can only be enabled in Firefox Nightly.

1. Enable the "fission.autostart" pref in Nightly using about:config. No need to mess with any other "fission.*" prefs.
2. Restart Nightly.

You can verify that Fission has been enabled by hovering over a loaded tab. If the tooltip contains a "[F]", Fission is enabled.

Disabling Fission

Opening a new non-fission window

If you encounter an issue while using Fission, it is possible to open a non-fission window within the same browsing session using the "New Non-Fission Window" item in the hamburger menu. This can be useful to determine if issues are Fission-specific, or to work around fission-specific breakage.

To disable Fission completely, reset the "fission.autostart" pref back to "false" and restart Nightly.

Milestones

Milestone	Description	Date
M1	OOP iframe framework	2019-02-28
M2	Semi-functional OOP iframe	2019-05-06
M3	Whistler All Hands demo	2019-06-26
M4	90% of mochitests are passing	2019-11-12
M4.1	100% of mochitests are passing	TBD
M5	Fission is dogfoodable (internal Fx user testing)	2020-05-29
M6	Enable in Nightly on desktop (X% rollout)	2020 H2
M7	Enable in Beta on desktop (X% rollout)	2020 Q4
MVP	Enable in Release on desktop (X% rollout)	2021 H1
-	Start work on Android Fission	2021 H1

Sub-pages

• Memory
• FrontEnd
• Enabling Tests with Fission
• Fix out-of-process uses of DocShell tree and nsIDocShellTreeItem