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). *[https://bugzilla.mozilla.org/show_bug.cgi?id=1565919 Bug 1565919] - Firefox in-built spellcheck for English en-US not working properly - single quote marks [70] *[https://bugzilla.mozilla.org/show_bug.cgi?id=1418629 Bug 1418629] - Spellcheck marks lost for words containing apostrophes [68] (<i>please do not comment in bug reports<br>https://bugzilla.mozilla.org/page.cgi?id=etiquette.html</i>)