I don't know if there is a direct way to have more than one spellchecker.

One thing you could try is to have more than one Firefox profile. Each would have its own spellchecker.

My reply may help you, but not me. Still, I think our issues are related but opposite. I am using Firefox 102, and spell check either English or French. In the current version, the spell check on the context menu allows for both languages to be checked simultaneously. When this is done, whichever language is written, the underline is gone. So, this may be good for you, but not me, and unless I know better, I suspect that this is a bug.

I understand that spellchecking now supports more than one language. I suggest you install additional dictionaries. Make sure you install dictionaries, not language packs, from the second link.

https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker https://addons.mozilla.org/firefox/language-tools/

You would have to check only one dictionary from the list and un-check the other(s), so only one dictionary is active at the time.

Thanks to all for clearing this up. I wasn't sure if two language spelling was an error. It is good for multilingual correspondence but for most of my interest, I can use only one language at a time because unknown words are added to persdict.dat. This is broken down by language, 'cleaned up', and added my Wiktionary list of missing words. https://en.wiktionary.org/wiki/User:Ineuw.