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

Enabling Fission

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

How to enable Fission:

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

Fission should now be enabled. To verify it's enabled, load a webpage, hover over the tab, looking for a "[F]" (i.e. Fission) in the tab's tooltip popup.

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.

In general, avoid modifying other "fission.*" prefs, as they are often for testing, or unstable features very early in development, and aren't ready to be used for day-to-day browsing.

To disable Fission, reset the "fission.autostart" pref 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 Q2
M6	Enable in Nightly on desktop (X% rollout)	2020
M7	Enable in Beta on desktop (X% rollout)	2020
MVP	Enable in Release on desktop (X% rollout)	TBD
-	Start work on Android Fission	~2021

Sub-pages

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