Firefox, while long being a poster-child for mainstream open-source software, along with OpenOffice (and my opinions on that have already been posted here), has also long been a telling reminder that the much-vaunted "many-eyes" bug-finding and fault-fixing of open source is not perfect.
The number of long-standing bugs in the various Mozilla code-bases is tiresomly large, and some of them really, really, suck.
Have you tried to print a multi-page document in Firefox lately, only to have a mere one page emerge from the printer? Have you scratched your head in bafflement, and tried again, and looked at all the settings, and found no way of getting the rest of it? No? Well, aren't you a lucky bastard then.
This, it seems, is a looooooooooong standing bug in Mozilla. Something about not being able to correctly interpret the contents of frames. Or absolutely positioned page elements. Or something. Whatever it is, it's fucking annoying.