Which version of Firefox are you using? If you're still using the original and not Firefox 2, you need to go to
www.mozilla.com and download the newer version. Also, Adobe Reader 7 should be upgraded to Adobe Reader 8. You are describing one of the problems that appears to be fixed with both of the newer versions.
I usually have installed the reader before Firefox 2 and that fixes the problem. If you are already using Firefox 2 I would suggest uninstalling it, going to
www.adobe.com to download and install Adobe Reader 8 and then reinstalling Firefox 2.