Unfortunately, there's no built-in way to change how Firefox spell-checks text. It might be possible for an add-on to do that, but I didn't see anything like that in a quick search.

What function is missing that you want to add? When you type nonsense into the reply box on this forum does Firefox mark it with a wavy underline? And if you right-click the marked word do you get suggestions at the top of the context menu? (The spelling suggestions are admittedly not as sophisticated as you would find in an Office suite, and probably not a smart as Google search, but you usually get at least one.)

If your language is not U.S. English, you may need a supplemental dictionary. See: How do I use the Firefox spell checker?

No...I'm not talking about spell-check, I'm asking about the spelling correction that is a function of Windows 10, and whether or not I can set it as the default for firefox.

Thanks for your reply, though. :)

Oh, does Windows 10 have "autocorrect" like on a mobile phone? I didn't realize the potential for embarrassing unintended words now extends to the desktop.

(I still use Windows 7. The closest thing is suggested word matches along the top of the on-screen keyboard available from the Accessibility programs.)

To use the native Windows feature, it seems that third party applications embed a Microsoft control (COM object) as their editor. Since Firefox is cross-platform and open source, that isn't really an option for Mozilla. If there is an open source autocorrect engine worth using, that probably could be integrated into or added onto Firefox's existing spelling feature much more easily.

Until then, hmm.

Actually, the FirefoxOS project included a keyboard with auto correction. That may have been built on top of Android or some other preexisting code. Perhaps someone could repurpose that logic into an add-on.

https://support.mozilla.org/kb/keyboard-settings-firefox-os#w_keyboard-tips_2

If you have Auto correction turned on, misspelled words will be automatically changed to the highlighted suggestion after you hit the space bar to move on to the next word. To undo an auto correction, just tap backspace.