Thunderbird won't start after dist-upgrade, hoary to breezy

[Matt Morgan]

I just tried to do a dist-upgrade from hoary to breezy. Almost
everything looks OK.

But Thunderbird won't start. When I try, I get a little dialog that says

---
Please Restart

Changes in the set of globally installed extensions detected.
You must restart Thunderbird one more time in order to finalize those changes!
---

There's an OK button, and the X in the upper right. Whichever I
choose, the box disappears for a second or so, then comes right back.
Thunderbird never starts.

I restarted X and then rebooted, but neither helped. Every time I
start Thunderbird, it's the same. I have to imagine this is some kind
of permissions error in the application directory or at least the
extensions directory, but they're all set to 644 or 755 (if a
directory), just like I'd expect.

Googling doesn't help much. Other people have gotten this error, but
it has either gone away after restarting, or they speak languages I
don't understand.

Any ideas?

Thanks,
Matt