Project Fission

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Fission is Mozilla's implementation of Site Isolation in Firefox. Site Isolation is a security feature that offers additional protection in case of large classes of security bugs. Site Isolation safely sandboxes web pages and web frames, isolating them from each other, further strengthening Firefox security.


Web security is designed in such a way that websites or webframes cannot access each other's data inside the browser. However, bugs happen. The Firefox teams and the Mozilla security teams invest considerable effort in avoiding security bugs, or, if they exist, finding them out and fixing them before release. However, if a bug somehow slips past developers, analysis and tests, and a sufficiently cunning attacker manages to find the bug before it can be fixed, they can sometimes craft a page specifically designed to access data from other sites that the user is currently visiting or has recently visited.

Firefox developers already employ a number of counter-measures to make such undetected bugs less likely to succeed, from programming in memory-safe languages to adopting defensive programming techniques. Site Isolation is a new counter-measure dedicated to this purpose. With Site Isolation, pages and frames are executed in processes dedicated to their origin.


Consider a blog on with a Facebook like button (frame from and a Twitter button (frame from Without Site Isolation, this entire page runs in a single process. If an undetected bug in Firefox somehow allows the main page of the blog to access data inside the frames despite the protections in place, the malicious owner of (or someone who had already stolen the domain) may be able to take advantage of this bug to impersonate the Firefox user in the Facebook and Twitter frames, and possibly use this impersonation to send fake messages or read private messages.

With Site Isolation, this blog now runs on three different processes, one for, one for and one for These processes are sandboxed which limits what each of the processes can do. Even if the malicious owner of were to take advantage of an undetected Firefox bug and to take control of the process in charge of, the processes in charge of or would reject any request from this compromised process. In other words, this hypothetical bug is not sufficient anymore to impersonate the Firefox user in the Facebook and Twitter frames.


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

Observing Fission

In desktop Firefox, you may open about:processes to see the processes used by Firefox.

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.

Enabling Fission

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

Fission was released to desktop Firefox in version 95.

  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.

You can verify that Fission has been enabled by hovering over the current tab. If the tooltip contains a "[F]", Fission is enabled. Background tabs' tooltips might not have the "[F]" if they are not loaded yet.

Disabling Fission

If you encounter an issue while using Fission, it can be useful to create a new profile in about:profiles and attempt to recreate the issue. This can be useful to determine if issues are Fission-specific. If you encounter an issue that only happens with Fission, please be sure to file a bug in Bugzilla about it as described above!

To disable Fission, set the "fission.autostart" pref to "false" and restart Firefox.