I don't have access to a Mac, but could you check your "Shrink to Fit" setting? I believe one way to do this is the print dialog -- change the pop-up from "Copies & Pages" to "Firefox" to get this full listing:

While you're there, does the paper size look logical?

Another way to check it would be the about:config preferences page.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the filter box, type or paste shrink and pause while the list is filtered

(3) Right-click the setting for the relevant printer(s) to toggle them from false back to true.

Does that make any difference?

Your first suggestion seemed to work. My dialog was slightly different from yours, but essentially the same. The shrink settings in about:config were all (2) true already.

Does the setting change in the print dialog stick, or will I have to go through this each time I print to pdf?

Hi mrn8tural, that setting should stick until you change it.

(By the way, I inserted that image from a help article, and it could be from an earlier version of MacOSX.)

I've had a similar problem with Firefox 13.01 on Mac Leopard (OS 10.5.8). I'd used the "Ignore Scale Shrink to Fit Width" in the Print dialogue box, and had even set the scale to 65% in Page Setup, which made the first page fine in the saved PDF file, but subsequent pages would be back at full scale and run off the page on the right, perhaps the top or bottom, too. (I didn't look carefully there.) After a number of intuitive attempts at solving this very annoying problem, I arrived here. Using the about:config method, I toggled both printers from true to false and back to true. Saved with "right click"/"control-click." Voila! First attempt--looks great at 100%! One language variation set me back a little. The "filter box" after going to "about:config" wasn't obvious, but I eventually put "shrink" in what I'd call the "search box." From then on it was a breeze.