Incredibly valuable, especially because it's not available for any other platform - only Mozilla. (Many thanks to the developers!) The word list is comprehensive. The suggested spellings seem to be consistent regarding words that are often spelled two ways (e.g. ones with a 'w' or an 'r'.) This guidance is helpful. The only thing that it might do better is to recognize slight misspellings. For example, maybe half the time when only an accent is missing, it fails to recognize a word at all and gives zero suggestions.
This review is for a previous version of the add-on (0.08).
This user has 2 previous reviews of this add-on.