To narrow it down to the add-on(s) causing the problem, disabling all of them under about:addons then re-enabling one at a time is a good approach, although can be time-consuming if you have a lot of add-ons installed.

Also - about:performance might give you some clues.

And on the add-on screen you will be able to tell which addons are compatible with multiprocess and weather it is enabled or disabled.

You may need to change a few extra things for multiprocess to work, like, accessibility features can block it from enabling.

(my post is now a bit redundant since I didn't refresh it to look for any new comments)

https://addons.mozilla.org/en-US/firefox/addon/add-on-compat...