I've used Grammarly for Firefox and have found it to be in the top 3 of the best programs I've used. It's unbelievably easy to use. I can't recommend it enough.

helpful, easy to install and to use.
a good help

Activated for all websites by default. You can exclude website one by one.
So, If you don't want Grammarly injected on every page/site and all your typing sent.
There's no option to deactivate by default and activate for only a few websites.
Probably no GRPD compliant.