Firefox doesn't like trailing apostrophes
Firefox does not like text contained in single quotation marks or trailing apostrophes.
Firefox - the peoples' browser.
I can get around this by istalling the Grammarly addon.
Additional System Details
- User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0
It shouldn't be a problem if a word is marked as misspelled. You can simply ignore this in case you are sure that it is OK.
This looks like a regression in Firefox 68 that is fixed in Firefox 70 Nightly (might possibly be back ported to 68 ESR).
- Bug 1565919 - Firefox in-built spellcheck for English en-US not working properly - single quote marks 
- Bug 1418629 - Spellcheck marks lost for words containing apostrophes 
(please do not comment in bug reports