'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...)
Additional System Details
- Shockwave Flash 32.0 r0
- User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0
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 
(please do not comment in bug reports