As of Firefox release 19 (released the week of February 18th, 2013), Firefox has now included a built-in PDF browser. While this does speed up the display of PDF objects, it breaks forms that must print to exact specifications.
Fixing this within Firefox is very simple, just follow these steps:
Open Firefox, open the Firefox Menu, and Choose Options.
Choose the Applications tab and scroll down to find Portable Document Format (PDF).
By default it will say “Preview in Firefox”. Change this to be “Use Adobe Acrobat (in Firefox)“.
Click OK to Save.