Open the site
about:support in Firefox, which indicates if e10s is enabled.
There is a line "Multi-process staged rollout", which would be set to
true if e10s is enabled. Also look for a number higher than 0 in the "Multiprocess Windows" entry (this is translated to your locale, e.g. "Fenster mit mehreren Prozessen" and "Web-Inhaltsprozesse" in german).
e.g., accessibility, add-ons can trigger disabling this feature.
On this site you can check, if add-ons, you are using are compatible: http://arewee10syet.com/. There is also a mozilla site to check compatibility, you find the addon-id in the extension-filenames in your profile folder, for example the "Tab Groups" add-on has the id
tabgroups@quicksaver and is incompatible with e10s.
If you would like to opt-in to test the feature anyway, open
about:config and toggle
true. On your next restart, e10s should be active.
If you've tried enabling e10s but about:support indicates that e10s is disabled (e.g., accessibility, add-ons can trigger this), you can force e10s on for testing purposes. Within
about:config create a new
boolean pref named
browser.tabs.remote.force-enable and set it to
true. This is not encouraged, use it at your own risk!
(I tried it anyway and it worked, even with Tab Groups Addon still enabled but it's risky!)