Update 2009-03-30: Here is a more reliable (but slightly more severe) way to work around this problem.
Have you ever tried to print something from Firefox and gotten an error message telling you that an unknown error occurred while printing? I have, and I know at least a few other people have had this problem. Apparently, this is an old bug, and the-edmeister has the fix:
type about:config in the URL bar and hit Enter
Preference name = print.print_printer
Right-Click that preference and select Reset, then restart Firefox.
See if Firefox is printing any better.
I usually can print one document after that, and I have to do it again to print another document. This solution is far from perfect, but it's good enough for now.