Words that you add to the personal dictionary are not included in the suggestions in the context menu, which I think is a slight limitation.

You could add a feature request if you like https://bugzilla.mozilla.org/

However, can't is in both the EN-US and EN-GB dictionaries, so it should be a suggestion.

Is there any way to replace the whole dictionary?

Replace it with what? If it's listed in the addons you can uninstall it. If it's not you can uninstall Firefox and reinstall it. What are you trying to achieve and how?

I thought maybe the whole dictionary was screwed up somehow. If you read my first post the problem I have is with some words with apostrophes. Try it yourself. Type the word cant just like I have it spelled here with no apostrophe. You will get the red underline telling you it is misspelled. If you right click on it to change it you get the suggestions can, cat, ant, etc. The correct word can't is not an option. If you misspell the word as in cannt you can right click on it and the correct spelling can't is available. This isn't a problem with all apostrophized words. If I type isnt without the apostrophe I can right click on it and isn't with the apostrophe is an available replacement.

If I type either cant or can't it is not marked because both are words.
If I type cannt then both cant and can't are suggested.
If I type isnt it is marked and isn't is suggested.
That is with either the EN-US or EN-GB dictionaries.

If that doesn't work for you maybe you're right - the dictionary is screwed up.

You could try installing it (again) as an add-on from here https://addons.mozilla.org/en-US/thunderbird/language-tools

The only possible complication is that if your dictionary is not shown as an add-on (my US one isn't but my British one is) I don't know if installing it that way will replace the dictionary or add it. But worth trying I think.