For.English this should be set up by default, but I'd double-check.
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".
I'm still puzzled by the bad spelling tool. At first I wondered why they didn't just grab the spelling tool from some other open source project, such as Libre Office. But then someone told me that they do use the same spelling tool that Libre Office uses: Hunspell. Hunspell is also used by Chrome and it is the built-in spelling tool on MacOS.
So then the reason it sucks on Firefox must be the dictionaries? But every correctly spelled word that Firefox tells me is misspelled that I've tried in Libre Office passes...so the question then is why Firefox doesn't use whatever dictionary Libre Office uses?