From MozillaWiki

To test out of process extensions, go to about:config and set extensions.webextensions.remote to true. Note: after flipping this pref it shouldn't require a restart, but it might not run reliably without one, so just restart Firefox ok?

APIs that are particularly affected by out of process:

context Menus

devtools

omnibox

tabs

permissions

webRequest

For more information see this etherpad