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 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
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.
関連：
・「Firefox」にサイト分離機能の計画--「Chrome」に続き - ZDNet Japan
・Site Isolation（サイト分離） is coming soon with Firefox 88 Beta
Site Isolation
Site Isolation, also known as Project Fission, makes sure each tab runs in a separate process. Also, it allows running a single page in different processes if it contains frames and modules from other domains. For example, running a page with a social media plugin will separate it into two processes: one is for the page’s domain, and another one for the plugin’s domain. The main idea behind Project Fission is to enhance browsing security by sandboxing pages and isolating them from others.
Currently, you can activate Mozilla’s Project Fission in the Firefox 87 Stable.
How to activate Site Isolation
Open Firefox.
Type about:config into the address bar.
Set the fission.autostart parameter set to True.
about:configで「fission.autostart」をtrueに変更してからFirefoxを再起動
