I wish there was a way to see what's in Thunderbird's dictionary and to be able to revise some of its entries.
For some reason, my Thunderbird dictionary thinks "don;t" is an OK word. No squiggly line appears below it to say, hey, you might want to re-spell this word.
I wish I could go in and take "don;t" out of my Thunderbird dictionary.
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You can edit the user dictionary, not the compiled on. This is basically the same way that Microsoft office works. There is a master dictionary for the language ( that you can select ) and a user dictionary.
In the compose/write window click on spelling to check spelling
Note the button to edit the Personal dictionary.
However I think the issue you raise is more than an edit of the dictionary. Mozilla spellcheckers (include Firefox here) do not cope with punctuation or numbers in the middle of text. THey simply ignore these obviously misspelled words.
My personal solution is to not use the Mozilla offering. I use this https://addons.thunderbird.net/en-us/thunderbird/addon/grammar-and-spell-checker/ I got there simply because at the tme it was the only ofering for Thunderbird and only use the free version. But it solves my issues with Thunderbird's spell checker.
Thanks for this. I found the user dictionary and made a few changes. I intend to add the spell checking extension you recommended, too.