Try the reset described here:
http://kb.mozillazine.org/Problems_printing_web_pages#Reset_printer

I tried to follow the instructions. After entering about:config I get pages and pages of entries (I don't know if these are the preferences). I went as far as page 30 without any mention of print-print_printer.

I went directly to print-print_printer and found out that my printer (HP Officejet 6500) is mentioned quite a few times.

The question is: what mistake did I make while following the instructions.

Thanks,

Renard

There's no "pages" of information, it's one long page of preferences.

print.printer will be listed multiple times, there are like 3 dozen or more preferences for each printer. If you have more than one printer there will be groups of prefs for each printer.

You can try resetting only print.print_printer and then restart Firefox, and see if that solves your problem. But chances are that you may need to reset every pref for that printer.

Also make sure that you don't have multiples of the same prefs for that printer. It is very possible that you may find duplicates like this or similar.
print.printer_HP Officejet 6500_Series(2).print_evenpages

(2) or (3) in a pref means you have multiple prefs for the same printer (usually caused by installing the print drivers multiple times without uninstalling the old drivers first), and that can cause problems with Firefox; the "printer" driver you select probably ain't gonna be the one that is really "installed". If you see something like that, you will definitely need to reset every pref for that printer; the duplicates will be gone after the restart and won't return after you use your printer the first time after the reset.

I am having the same problem. I also have the same

 printer, HP 6500 plus wireless.  Could this be a printer problem, at least when using Firefox?