That's a good question. I'm not sure if there is a better way but I would just delete it from <profile>/extensions. You can find <profile> by going to about:support and looking for "Profile Directory" (6th from the top for me).

Knowledge Base info of uninstalling xpi http://kb.mozillazine.org/Uninstalling_extensions#Uninstalli...

`about:studies` will show active studies and allow you to remove

I am not sure on others, but it does not show in "about:studies" on mine since I manually added it.

Same.

I unistalled it and my add ons are still working!