If about:support (not about:config) says Enabled by user instead of Enabled by default, that means something in your environment is overriding the browser.tabs.remote.autostart.2 preference in about:config, or at least causing it to get toggled back after restart. Are you saying the preference behaves like it’s not getting saved at all? I’m almost sure it should just stay as set, or something might prevent it from getting saved.

Note that in order to disable e10s, browser.tabs.remote.autostart also needs to be set to false. Also, the browser.tabs.remote.autostart.2 preference is ignored and overridden by browser.tabs.remote.force-enable if that may have been added in the past and set to true (?). However, both autostart preferences should still display as false in about:config in that case, and about:support should display "Enabled by user". I’m not aware of any other preferences overriding or even reverting these changed ones, but there may be some. (Generally it’s harder to force enabling e10s than disabling it, as could happen with add-ons or accessibility.)

If there is no browser.tabs.remote.force-enable preference involved, perhaps you could:

- Double-check all methods used for modifying preferences - Use the current profile’s prefs.js file and test the same on a non-corporate system, or just look at the file after changing browser.tabs.remote.autostart.2 after closing Firefox to see if it gets saved properly - Quickly test with a new/fresh profile and compare both profile’s prefs.js files if needed - Have a look at this question, as well as this bug

If changing the preference manually works, I think deploying it in a js file should work as well.

Thanks for the answer.

From article Firefox in an enterprise environment I created:

file #1 - "C:\[FF installation folder]\defaults\pref\[config-pointer].js" contains: // Any comment. You must start the file with a comment! pref("general.config.filename", "[config-file].cfg"); pref("general.config.obscure_value", 0);

file #2 - "C:\[FF installation folder]\[config-file].cfg - for example: // Any comment. You must start the file with a comment! pref("security.enterprise_roots.enabled", true); pref("browser.tabs.remote.autostart.2", false);

Yes, file [config-file].cfg must be in main FF folder. If you have more .js files in "defaults\prefs" which contains the same value settings (like pref("security.enterprise_roots.enabled", true);) - delete it and put into .cfg file.

There is a browser.tabs.remote.force-disable pref that you can try to lock to true.

• lockPref("browser.tabs.remote.force-disable", true);

Note that Firefox comes with the e10srollout extension in the browser/features folder that might override/modify some settings at a later time.