The people who answer questions here, for the most part, are other Firefox users volunteering their time (like me), not Mozilla employees or Firefox developers.

If you want to leave feedback for Firefox developers, you can go to the Firefox Help menu and select Submit Feedback... or use this link. Your feedback gets collected by a team of people who read it and gather data about the most common issues.

You can add unknown words that are flagged as misspelled to the persdict.dat file in the profile folder via the right-click context menu (Add to Dictionary).

Thank you FredMcD for your suggestion to send feedback. I often substitute "rgw" or "yjr" for the word "the," by being one key right or left of the intended key, and I've often wished "the" was among the choices, to take one example.

I do use government agency acronyms or technical terms that if misspelled won't have correct offerings even if I add the correct spelling to the personal dictionary. And I am a poor typist.  :)

Thank you, COR-EL for your reminder about the personal dictionary.

Unfortunately Firefox won't offer corrections that are part of the persdict.dat file. You can only use persdict.dat to prevent flagging words that aren't part of the selected dictionary. Only words in the selected dictionary file are suggested.

I don't think that Hunspell is able to provide advanced suggestions for misspelled word like characters typed on the keyboard that are shifted from their position. Only when a word still has some association with the intended word then it is possible to provide a useful suggestion.

A search and replace text extension or suitable bookmarklet could handle such cases. The same for cases where you accidentally pressed the Caps Lock button.