A two part solution:

1) Find and on a "PDF document"
"Properties"
"Open With" tab
You should see "Document Viewer" and possibly other PDF viewers with
radio-buttons
Select your poison.

2) In Firefox
"Edit"
"Preferences"
"Applications" tab
In Applications there are two columns
On the Left is "Content Type" and on the Right is "Action"
Scroll down until you see "PDF" and it
on the down-arrow in the "Action" column associated with "PDF" (mine is
"Always ask")
Pick your poison. If you choose "Use Document Viewer (default)" this is the
viewer set-up in Step 1.

I haven't tried Step 2 so if we luck-out or strikeout let us know but it sounds
good. (I know from experience that Step 1 works.)