Haven't met this myself, but on the Firefox help forum it was reported removing the add-on "Printing Helper 2.5" cured the problem. The user who discovered this didn't recall installing the add-on; I'd looked for any add-on that doesn't belong, since the malware creator could easily change the name of the fake add-on.
Firefox has an option "Restart with Add-ons Disabled", it's under Help. Always try this as a diagnostic step, you may need only remove an add-on to fix a problem.
Keep Up with TechRepublic