You have 2 Adobe Reader (PDF) plugins listed in the information that was included with your original question (click "More system info..." to the right of your original question):
Adobe PDF Plug-In For Firefox and Netscape
Adobe PDF Plug-In For Firefox and Netscape "9.4.1"
The first one is probably a very old version and is probably located in your C:\Program Files\Mozilla Firefox\plugins folder. If you see a file named nppdf32.dll in that folder, either rename, move to another location, or delete that file.
The information submitted with your question indicates that you have out of date plugins with known security and stability issues that should be updated. To see the plugins submitted with your question, click "More system details..." to the right of your original question post.
Adobe PDF Plug-In For Firefox and Netscape "9.4.1" (see above re: duplicates)
New Adobe Reader X (version 10) with Protected Mode just released 2010-11-19
Choose the latest #.x version (example 9.x, for version 9)
Choose the highest number version listed
NOTE: 10.x is the new Adobe Reader X (Windows and Mac only as of this posting)
Choose your language
Download the file
Windows: choose the .exe file; Mac: choose the .dmg file
Using either of the links below will force you to install the "getPlus" Adobe Download Manager. Also be sure to uncheck the McAfee Scanner if you do not want the link forcibly installed on your desktop