It's no longer working, using Firefox 93.0. I don't seem to reply your msg, so let me edit this. Basically when you highlight some contents and click on the grammar add-on icon, it just circling and circling and never complete anything. I tried in the private mode, the same behavior. Safe-mode won't help because it will disable all the add-ons including this one.

too difficult and slow to use

Very useful in French and in English (But I would like to choose no English variant, a "global" English would be useful for non natives who want to speak "any" English)

This worked for a short period of time then just stopped. When I'd go to check it just keep trying to load. For the short time it worked it worked ok. I use grammarly for everything except twitter bc it's not supported on twitter so I added this one for twitter. Wish there were more choices for grammar checkers.