Thunderbird spellcheck does not mark some misspellings

When using spellcheck in Thunderbird, I can add a word into custom dictionary, so that Thunderbird does not consider it as misspelling next time. But how can I do the opposite? For some reason, Thunderbird believes that "thew" is an English word, and does not mark it as misspelling (underline) it. "thew" is quite frequent misspelling in my case when I type "the". I checked Cambridge dictionary, and "thew" is not a real word in English. Why does Thunderbird not correct it? How can I fix this ? I am using Thunderbird 68 on Debian 10. I am not sure whether Thunderbird uses aspell or hunspell. I have both installed. How can I check which spellcheck library I am actually using? **EDIT AND UPDATE** The English dictionary in Thunderbird I am using is English (United States). But regardless whether you consider thew a proper English word or not, this was meant as an example. I am interested in a solution where I can tell Thunderbird to mark given word as misspelling, where it previously considered it a legit word.