That's a good question. I believe Adobe Reader acts as it's on browser for PDF files.
If you wish to view PDF files in Firefox, I highly recommend getting a plug-in named "Web to PDF." I've been using it for a few months now and I've never had a problem with it.
If you do a search for "Web to PDF", with or without the quotes you'll be able to find it.
Or you could just go to the Mozilla Firefox site and check out the plug-ins, you'll find it there.
After installing, just right click (on your mouse), on the PDF icon or link, select "open with", then choose "Mozilla Firefox" and that should do it.
I hope this helps! If you have any problems, let me know and I'll see what I can do.
This same problem appeared yesterday after I removed an earlier (7) version of Acrobat. I also had Reader 9.3 installed. I use Firefox as my browser on a geriatric Dell desktop running XP. The solution I found is this - I removed 9.3, then installed Reader 8.2. This worked fine, so I then re-installed 9.3. This also worked fine, and automatically updated to 9.3.1, which is working as it should do. Why this problem emerged in the first place, I have no idea, so try clearing out the old version of Reader, then install the most recent version on a 'blank canvas'. Hopefully that will fix the problem for you, as it did for me.