Thanks for the advice, but /usr/lib/firefox is messed up too. apt-get won't install firefox-bin to it, and it looks like it's missing some other files in that folder. I downloaded firefox from mozilla.com, and then I tried copying that folder to /usr/lib/ but it won't run from there. I says Segmentation fault (core dumped) when I try to run it from the command line, and just doesn't do anything when I run it from nautilus. But, I can run it from my home folder, so I guess that's good enough.