- Enter the discussion forum, and select the threads.
- Click Collect.
- Click Print Preview
- A new window will open with the option to print. Select OK.
- Type about:config in the Location Bar and press Enter. A list of preferences will appear.
- Find the preference print.print_printer (Note: this preference is missing in Firefox profiles where you've never used the printer.)
- Right-click (context-click) that preference and select "Reset".
- Restart Firefox and see if the problem is resolved.
If you get the warning dialog, "Printing failed when starting the document" and if the print.print_printer preference has no printer shown, either you don't have a printer selected, or Firefox doesn't know it is installed on your system.