I am not aware of any that work with current versions.

wow the forum dug up this old thread from 2018. It appears nothing has changed. https://support.mozilla.org/en-US/questions/1226880

I don't know what version of Firefox or T-bird I had, but on my old computer with Windows 7, T-bird had a thesaurus. I used it all the time. After I bought my new computer in April with Windows 10, I noticed there was no thesaurus. I really miss having it. The FF version I have now is 91.0.2. The version of T-bird I have now is 78.11 (although since being on it this afternoon, I noticed it updated to 78.13; the next time I turn on, 78.13 will be in action). So why would FF/T-bird do away with the thesaurus? It's one of the most useful features. It's like doing away with the spell check.

I don't know what version of Firefox or T-bird I had, but on my old computer with Windows 7, T-bird had a thesaurus. I used it all the time. After I bought my new computer in April with Windows 10, I noticed there was no thesaurus. I really miss having it. The FF version I have now is 91.0.2. The version of T-bird I have now is 78.11 (although since being on it this afternoon, I noticed it updated to 78.13; the next time I turn on, 78.13 will be in action). So why would FF/T-bird do away with the thesaurus? It's one of the most useful features. It's like doing away with the spell check.

You may have had an addon, but neither Thunderbird or Firefox has ever had a thesaurus natively.

Since July when this thread started I have become aware of the Grammar and Spell Checker (it was ported Thunderbird on the 26th of July) and offers a thesaurus component. But that is it.

https://addons.thunderbird.net/en-US/thunderbird/addon/grammar-and-spell-checker/?src=ss