Last night I got it into my head that I needed to update Firefox NOOOOOOW. Normally, I'd read through the instructions and follow them, but this time.... nah, too easy. Just uninstall the version I'd been using (0.9) and install the new version (1.0).
Well, apparently, you're not supposed to do that. First, you're supposed to uninstall the extensions and themes, THEN uninstall the program to prevent conflicts. Which I didn't do.
Long story not-as-long, I installed 1.0 and it opened, but wouldn't reopen. Uninstalled it. Reinstalled 1.0. Wouldn't work. Uninstalled it. Installed 0.9, and it worked. Reinstalled the extensions it couldn't find on the initial install. Wouldn't restart. *sigh*
The solution I found seems fairly simple - install it in a different directory. Which I will be trying when I get home.
(Update: it worked!)