Thunderbird doesn't have and never had a native "auto-correct" feature. Long times ago there had been an add-on "TB Auto-correct 0.0.6.2" written by Frank DiLecce in 2005 which I actively used later https://forums.mozfr.org/viewtopic.php?p=720079#p720079 It had a very limited thesaurus which could be modified/completed by the user, a bit like the personal dictionary in modern TB versions. Later it was replaced by "mms Auto Correct" which worked until TB 52...

If you only want to activate the Thunderbird spell-checker feature (which is not the same as "auto-correct) you have to go to Settings > Composition > Spelling and check ⬜︎ either "Enable spell check as you type" or "Check spelling before sending". But the correction of spell check errors will still have to be executed manually....

With the Windows version, Tools | Settings | Composition | Spelling, you have two options that control the defaults of that feature, "Check spelling before sending" and "Enable spellcheck as you type." You can also temporrily enable/disable "Spellcheck as you type" on a case-by-case basis from the Options menu in the Write window.