I switched over to Fire Fox 3, and now it won't load PDF files. Is this a common issue? How can I fix it?

Did you try reinstalling adobe?

Yeah, I'd try that. Flash crapped out on me and it took two installation attempts to get it working.

It told me I had to remove it first and now it won't install again.

try getting a pdf installer add-on. FF has always been bad at handling pdfs, IMO. Either get a download manager specifically for firefox, or just save all your pdfs directly, without opening them in firefox.

Try foxitreader...its faster and uses less memory. It integrates into any browser.

 

http://www.foxitsoftware.com/pdf/rd_intro.php