As I moved to FireFox version 3.0 several months ago, I started having problems with FF freezing when trying to open PDFs. This problem didn’t occur EVERY time, but often enough to be really annoying. And it seems to take FF/Vista a really long time (several minutes) to clear this freeze. Now at FF 3.5.x, this problem has become unbearable.
The Fix for PDF Downloads in Firefox
There is an addon (of course!) designed to solve this problem. Download and install PDF Download 2.2.0.2 or more recent.
Use PDF Download to do whatever you like with PDF files on the Web. Regain control of them and eliminate browser problems, view PDFs directly in Firefox as HTML, and use the all-new Web-to-PDF toolbar to save and share Web pages as high-quality PDF files.
Configuring PDF Downloads
You need to set a couple thing in the Option dialogue for this addon to work properly. Go to Tools | Addons and select the Download PDF addon. On the Genral tab, select Open PDF. Then on the Opening PDF tab, select “Use OS default viewer”. Now your PDFs will download and open in your default PDF viewer. In my case, this is Foxit Reader.
This addon works great! And no more freezing! 
