I upgraded to windows 7 about 6 months ago (yes, I was an XP clinger). After the upgrade (a very painful process-thank you Microsoft), occasionionally I would get the above error message when trying to print from Firefox. Back then it was likely FF v. 46.something, maybe 47 or 48. Now, with 50.0.2 it's happening all the time. The printer in use is a multi-function Sharp copier/printer on the network. Funny thing, if I print it to a pdf using PrimoPDF, I can then print from my pdf viewer program. Of course that adds time to the process. So, just in-case, I deleted the printer from Windows and re-installed it to rule out some funky driver issue. That did not resolve the problem. Printing always worked from FF on XP (see, clinging or it's not broken, so don't fix it, isn't necessarily bad).