'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...)
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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)