'Check Spelling' indicates an error when an ending single quote is used (like the quote after Spelling).
A properly spelled word, when followed by a single quote ('), indicates a misspelling (wavy red line). Removing the trailing quote 'fixes' this. (as shown, 'fixes' has the wavy red line, 'fixes is okay...)
Wšě wotmołwy (1)
Known issue that will be fixed in a future Firefox version (69 to be released Sep 3).
- Bug 1565919 - Firefox in-built spellcheck for English en-US not working properly - single quote marks [70]
(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)