Displaying PDF files in a browser other than Safari requires you to have Adobe's Acrobat reader installed.
Acrobat Reader installs a plug-in for each browser it finds on your system.
So, if you install Acrobat Reader, then you will be able to view your PDF files.
Now, you also need to know that Adobe has not updated their software to be 'Universal' (compatible with Intel Macs).
This means that FireFox (and any other browser) will automatically disable PowerPC plug-ins while you are using them in Intel Native Mode. So, you will need to run FireFox in 'Rosetta' mode. This will enable the PowerPC plug-ins to work.
Here's a document explaining how to force a Universal program (such as FireFox) in Rosetta / PowerPC mode:
http://docs.info.apple.com/article.html?artnum=303120
So, run FireFox in Rosetta mode and Install Acrobat Reader (the latest version) and see if that works.
As for your other Adobe program, Adobe still has not updated it to run Natively on your Intel Mac, so that is likely part of your problem.
Let me know if you have other questions, or need further clarification on some of the details.
I hope this helps.