I submitted a Thunderbird bug 13 years ago now, that Thunderbird doesn't let you just search for a word, and find all copies of that word: https://bugzilla.mozilla.org/show_bug.cgi?id=752844 . Their search instead tries to be clever, but it then finds stupid stuff like (in my case) "The base of Wedding is wed. The plural of wed is weds. Lets return every email ever from a Wednesday"

Still not fixed.