With Firefox 49.0.1 on OS/X 10.11.6 (15G1004) the "Mail PDF" option in the PDF pulldown in the Print dialogue box does not work (it prints to the printer). The option works for other applications. This is a new Mac (about two weeks old). I used Print->PDF->Mail PDF all the time and it worked fine with Firefox on my old Mac. I believe this is the first time I have tried to use this feature with the new Mac. I do not have my old Mac available, so cannot check the OS version or the Firefox version, but I believe both were up to date on the old Mac. The error is repeatable. I tried three times and it printed to my printer. I switched to a printer that was not available and tried it two more times and it queued the job to the printer I had selected. The attached screen shot shows the dialogue box. I do know how to capture the pulldown menu because the screenshot command I use does not record those elements of a dialogue box. The pulldown is at the lower left of the dialogue box. I select the fifth item "Mail PDF". Other options in this menu that save a PDF to a file or open a PDF in Preview work. Those are the only other two I have tested (I only use the three options in the PDF pulldown).