Spell Check errors
This seems to be a problem that has gone on for years. Thunderbird spell checker does not correctly handle ordinal numbers (1st, 2nd, 3rd, 4th, etc.). There are bug reports that say it was fixed. If it was, the fix apparently was undone along the way. It definitely does not work in Thunderbird 91.7. I am using the default English (U.S) dictionary.
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What is the exact problem? Ordinals are not flagged as misspelled here in TB 91 and the En-US or En-CA dictionaries. But I notice that any string that begins or ends with a number, such as 1sd or 4tu or st1 or asdf3, are not flagged.
I've attached two screenshots--one showing that it is flagging "nd" as a misspelling because it does not recognize "2nd" and the other showing "th" being flagged as a misspelling because it does not recognize "4th". It does not seem to have a problem with "1st" or "3rd".
I agree, it should suggest ordinals for nd, rd, th and st, at least for 1-9. Contact the dictionary add-on developer and ask why. Or, perhaps you can find a related bug here.