Firefox's latest update includes a built-in pdf viewer that doesn't seem to work. You can do the following to change the default viewer to Adobe or whatever else you use:
In the Toolbar, click the Firefox box. Click on Options. Select the Applications Tab. Scroll down to Portable Document Format (pdf) option. There should be a drop down. Select Adobe (if you are using something else, select the "use other" and then browse to find your viewer). Then click ok. You should now be able to open PDF files. It will take a second or 2 for your file to load.
Hope this helps.