A while back, I had trouble printing from Firefox. I had an unreliable solution that worked for a lot of people a lot of the time. It didn't work all the time, though, and it didn't work for everyone.
The best solution is simply to uninstall Firefox, delete your Firefox profile, and reinstall Firefox. Sync your bookmarks ahead of time with Google Toolbar/Google Bookmarks or FoxMarks if you don't want to lose them. I was initially too stubborn to clean up and reinstall Firefox, but after having done it, everything works better. I am a happier man and a better citizen for having done it. Give it a try.