I admit that I skipped safe mode – I just ran it from the terminal since that was working anyhow, and adjusted my addons that way.
In case this comes up in a search for anyone else, it *seems* to have been the HTitle extension. (All apologies to the dev of that addon if I’m wong.)
I wasn’t very scientific about it - I disabled that extension (the most recent one I had installed) shut down firefox, and saw that it would then launch normally.
Since then, however, I’ve found that flashgot is making FF freeze up if I right click, so I’ve had to disable that as well.
I wonder why the sudden issues…
In any case, I’m in good shape now.