On the Mac, many users have to disable the Adobe Acrobat plugin to solve the blank page problem. You can do that here:
Tools > Add-ons > Plugins category
If Firefox can't figure out to launch PDF forms in Preview, you can educate it here:
Firefox > Preferences > Applications
In the slim search box above the list, type or paste PDF, then pause while Firefox filters the list. You should be able to update the program to handle the PDF using the drop-down list in the right column of the list.
Not sure about the Word documents. If you search the Application preferences in that same dialog for Word or DOC, can you figure out how Firefox is trying to open them?