I've also tried selecting "Add dictionaries...". The page that goes to says that there are two available for my locale.

These are "English (US) Language Pack" [1] and "English United States Dictionary" [2]. I have installed both of them.

Those do show up as enabled under "Languages" and "Dictionaries", respectively, on the about::addons page.

This does not change the options available in the spelling context menu. It only contains "English (United States)" as it did before adding that dictionary and language pack, and there is no change in the results on my test words [3].

[1] https://addons.mozilla.org/en-US/firefox/addon/english-us-la...

[2] https://addons.mozilla.org/en-US/firefox/addon/us-english-di...

[3] Here's my current test words that Firefox incorrect says are misspelled: prosecutable subtractive tunable epicycle inductor subparagraphs transactional micropayments blacksmithing inductor solvability verifier ethicist tradable tradeable auditable splitter surveil responder commenter. The other things that I know use Hunspell (MacOS, Chrome, Libre Office) get all of those right except for "tradeable".