Note to self: before trying out any plugins, clear cache and make a backup of ~/.mozilla/firefox/*.default, lest the plugins muck about unexpectedly with Important Stuff.
I don't know if it was FoxyProxy or Torbutton, but something messed up some combination of cookies, form history, and saved passwords (I'm not sure exactly what). Also, "Clear search history" is grayed out.
Hm. Saved passwords are still saved, according to the saved password editor... but they don't work. Grrrr.
Maybe I should back off on the whole notion of (selectively) proxying Firefox, and have a different browser set to use Tor.
Update: looks like Torbutton had set my Firefox preferences for full-paranoid, turning off most history and unchecking "remember passwords". Put history and password settings back to normal, and things start working again.