I'm not sure I understand what you are seeing, but it sounds as though instead of getting the website's specified fonts, or normal fonts like Arial and Times New Roman, you are getting a blocky font in Print Preview. Is that what you mean?

And does that occur for all sites or only particular sites?

If you go ahead and print, does the font appear normal or does it also have the same problem?

Or is it a problem with the paper size?

Print preview shows page size as the ebay label I use with my label printer. It also prints to the label format on my regular printer. Label printer is not default printer.

Unfortunately, I think Preview is going to lay out the page for the last used printer rather than your default printer. If you check the print dialog (either using the Print button in Preview or by pressing Ctrl+p) is Firefox selecting your label printer when this happens?

I don't know of a convenient way to switch the selected printer. In other words, I think the only way to switch the printer through the interface is to print to it.

In about:config, the print_printer preference usually will match the last used printer. Is that what you see?