It is possible that Outlook uses its own spelling check dictionary and disables the Firefox spelling checker.

A webpage can disable the Firefox spell checker via a spellcheck="false" attribute of an input field or textarea or editable element and possibly provide its own spelling checker. You can right-click in the text area or input field and use "Inspect" to check in the Inspector if this text area has a spellcheck="false" attribute. You can remove this spellcheck="false" attribute or change it to spellcheck="true" via a double-click or via "Edit As HTML" to make the "Check Spelling" item in the right-click context menu reappear.

I checked to see if Outlook has a spell check, but they only have a Microsoft Editor option, which I have disabled. I tried the reset to default option for Microsoft editor and now I see it's built in spell/grammar checker is enabled and I can't disable it anywhere. The body of text shows spellcheck="false" after about one minute of the webpage being open and I'm typing it up. Inspecting the body of the email and changing it to spellcheck="true" resolves the issue. If I refresh the page, it happens again. Is there a way to make this permanent?

Yes, something has changed recently in Outlook. The spell checker highlights the word and then you click the highlighted text once to get suggestions. see screenshot On this forum the right click option for spell check is there.

I've had this problem for many months, but I finally wanted to look into resolving the issue now. It looks like when I clicked the reset to default under Microsoft editor, it brought back the built in spell check editor option in the body of the email. The correction and spell check toggle option isn't listed anymore.

I turned it off, log out and back in, but the Editor was still working even with the option off.  ;-))

Good luck with this one...