In the early days of Firefox when the add-ons were first introduced, when you install an add-on, you need to restart Firefox browser for it be installed. Nowadays, most extensions available on AMO are restartless and with webextensions, their number has been increased. Because of this Mozilla now showsRestart required’ badge for add-ons to indicate that add-on requires a restart.

MultiProcess Firefox requires a browser restart even to install restartless add-on

But when you install a restartless add-on in current Firefox that has MultiProcess or e10s enabled, you need to restart the browser, do you know the reason why?

It is to turn off multiprocess because initial phase of e10s only work with no add-ons installed, strict rules!. What happens after the restart? e10s will be disabled and when you visit about:support page, multiprocess Firefox line shows 0/1 (disabled by add-on) message.