Some extensions store data in indexedDB (storage/default) in the Firefox profile folder and I'm not sure whether this gets cleaned up in all cases assuming that you haven't selected to keep this storage.

about:config ->

• extensions.webextensions.keepStorageOnUninstall
• extensions.webextensions.keepUuidOnUninstall

What you should do is use something like CCleaner to insure nothing is left as well but know what your doing when you use this program as it does a good job and too good sometimes removing stuff. B

Ccleaner does not do a good job as it can cause more problems that help with the Mozilla related products including desktop Firefox. This is from what I have seen from doing support all these years. None of the regulars here or on mozillaZine recommends using Cclearner to clean anything Firefox related.