Try resetting your printer margins in about:config.

Type about:config in the URL bar and hit Enter.
If you see the warning, you can confirm that you want to access that page.

Filter = print_margin

Right-click and select Reset for both margin_left and margin_right for that particular printer, and then restart Firefox. If you have ever used other "printers" in that Firefox Profile, you'll have separate groups of preferences listed for each printer.

Okay - it seems like we're almost there... I'm resetting, but the problem doesn't solve.

How to I restart Firefox - close?

Close Firefox and re-open it again. Settings like that aren't saved until Firefox is closed.