I had a problem printing a PayPal label w/ Firefox. The label showed up in the print box, but the printer only printed the receipt, not the actual shipping label. Doing a re-print solved the problem.

USPS not much better. Label would not print not matter what I tried. Had to void the label and take the package to the window at the post office. Thank goodness the label was only a few bucks that I'll have to wait several weeks to have returned to my account.