I think I figured it out and it's not adblockers or add-ins. I reported this using MS Edge, which allows every damned thing to infect a machine. So I switched to Librewolf, which blocks so much that most websites down't work, and there it was fine!
Then I went back to Edge, looked at a post I made in Librewolf with bold text, and only then I could see that I was somehow in BBCode Mode. And then I was able to switch it off. So, it's BBCode Mode that's actually glitchy. Somehow puts the editor in a state where BB tags aren't visible, and the editor buttons are all disabled, so the editor's effectively deadlocked.
Too frickin weird.