Could you describe the Gmail connection a bit more: you are printing a Gmail message that has a PDF attachment? And when you choose print, first Firefox prints the HTML parts, and then Acrobat prints the PDF attachment??

1.I open an email with a PDF attachment 2. I click on the document to preview it 3. At the top of the document preview window (within GMAIL/FIREFOX) I choose the print icon that pops up (The icon choices are "Open", a down arrow, google drive symbol, print symbol, download symbol. These all appear in the top center. 4. The document opens in a new tab in Firefox and the adobe print dialog box pops up. 5. I either choose "print" or "cancel" 6. A second dialog box pops up, repeat step 5

Note: This also happens if I change the applications option to open PDF's with Adobe Acrobat, instead of with Adobe Acrobat (in Firefox); the difference is that at step 4, the document opens in the Adobe Acrobat program.

Hmm... yes... Google is doing something interesting here.

Your idea of launching it externally provided some new information, which is that the file clearly is getting modified by Google during this procedure of selecting to print it.

The first screen shot shows the major change in file size.

The second screen shot shows that the "print" version triggers a JavaScript warning (in my configuration) while the "download" version does not.

Whatever Google is adding into the document seems to be triggering printing twice, but if it only does this on Firefox, that's really puzzling. And also I can't think of a way to fix that.