I think the easiest way is to create your signature as a graphic image, jpg or png, which includes the word hən̓q̓əmin̓əm̓. See attached picture.

That's an okay solution for signatures, but don't most email readers block images by default if they're from unknown senders? And if I'm using the name of other First Nations peoples in the body of an email, your suggested solution doesn't resolve that concern either.

The topic of my thread asks specifically about words with non-English characters in personal dictionaries, not this one word in my signature file--which I only referenced as an example.

Thank you for thinking about this, though.

Most mail clients block remote images by default, i.e. those that are hosted on remote servers, but don't necessarily block images attached to messages.

I can't answer why the spellcheck doesn't parse a word like hən̓q̓əmin̓əm̓ correctly, even if it's in the personal dictionary, but I think it's asking a lot to expect any system to recognize characters for every obscure language.

To avoid spelling errors, you could use an add-on like QuickText to insert words that are difficult to remember, and just ignore spellcheck flags for those words.

I can't answer why the spellcheck doesn't parse a word like hən̓q̓əmin̓əm̓ correctly, even if it's in the personal dictionary...

Fair enough. Thanks for attempting to answer the question.

...but I think it's asking a lot to expect any system to recognize characters for every obscure language.

I thought that if there's a set of characters that a person uses often while writing emails in Thunderbird, if you put that entire set of characters into your personal dictionary then when spellcheck does its thing, it would ignore the entire set as a whole word rather than break it out into component parts (and also skipping one component, too). Shame that this isn't a thing in Thunderbird, and I'm surprised that it's not possible.