In case your printer settings have become corrupted, try the quick fixes in this thread:

Recently unable to print from Firefox after update-- can print from everything else. | Firefox Support Forum | Firefox Help

Thanks jscher2000, I tried resetting all entries for my printer. After restarting Firefox the same thing happened. Must be something else.

It seems there are lots of problems with Firefox 4.0.1. I guess I will just live with my problem.

My default printer is a PDF printer driver. Not practical for everyone, I know, but if you aren't going to roll back to Firefox 3.6, it might be useful for you when you need to print.

In my situation, Firefox (4.0.1) appears to set the paper size for the printer to a very small size ("Organizer J") at intervals. Consequence is a 20 page printout on letter size of a 2 page document. Have not yet isolated when/why this occurs, BUT, it has happened after letter size has been selected, successfully used, and successfully re-used.

If nothing else, were Firefox software team to ensure default paper size selection is reasonable according to tenor of system / print driver that would be useful and helpful.

@breeve, I think that might be one of the glitches addressed in this MozillaZine knowledge base article (unofficial but very useful site):

http://kb.mozillazine.org/Problems_printing_web_pages

Try the first step (reset printer setting) first.