Project Fission

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Fission is Mozilla's project to implement Firefox site isolation. This introduces a major architecture change in Firefox such that a unique process is allocated for the main web page and for each unique third-party iframe embedded in it, thereby safely sandboxing them and further strengthening Firefox security.

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.

Known Issues

  • Some extensions might not work fully. But please file bugs if you find an extension that doesn't work!
  • 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.
  • Linux resource exhaustion issues with large tab counts, including:
    • Excessive memory usage
    • File descriptor exhaustion issues
    • X11 connection exhaustion on Linux (bug 1635451)

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. In about:config, set the "fission.autostart" and "gfx.webrender.all" prefs to "true". DO NOT edit any other "fission.*" or "gfx.webrender.*" 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, reset the "fission.autostart" pref back to "false" and restart Nightly.

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