I wonder if it'd be possible to write an extension that uses the Native messaging API (https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web...) to talk to a lightweight shim that uses the macOS native spell checker (https://developer.apple.com/documentation/appkit/nsspellchec...) instead. It'd be faster than sending it to a cloud service, yet still much better than the Firefox one.

Has anyone tried that, or know any reasons that shouldn't work?

I might try to write one, otherwise.