This has been working fine for a while, but I had to refresh Firefox for other reasons.
Then it is necessary to re-install the add-ons.
After installing Grammarly, Firefox could not get to any web page I requested.
After disabling Grammarly, Firefox would work without a problem.
Firefox does have a spell checker so I will stick with that to minimize having too many add-ons.

Running Firefox Quantum 61.0.1 (64-bit).

When I log into my PC as a different user they both play well together. Go figure?