Note that Firefox provides its own reader per default. You may have to deactivate it, if you prefer the plugin-based alternative :

View PDF files using Firefox’s built-in viewer

Other than that, please check if all your plugins are up-to-date. To do this, go to the Mozilla Plugin Check site.

Once you're there, the site will check if all your plugins have the latest versions. If you see plugins in the list that have a yellow Update button or a red Update now button, please update these immediately.

To do so, please click each red or yellow button. Then you should see a site that allows you to download the latest version. Double-click the downloaded file to start the installation and follow the steps mentioned in the installation procedure.

Tried everything mentioned above except for checking plug in status with no change. Mozilla has been checking plug in status for about ten minutes now with no results so far. May have to restart that function.

I haven't done anything else to attempt to fix the problem and today the problem is gone. I have no idea what caused the problem and no idea why it has gone away.

I don't know if this is any help I have had a problem where the pdf files were not opening correctly which I think was the problem here. I spent hours of head scratching and have now found out what the problem is. Right click on the Adobe Reader Icon on the desktop. Select properties and then the compatibility tab. You will see the uppermost section in this screen is Compatibility mode. I have Windows 7 and mine was set to Windows XP. I changed that to Windows 7 and now the problem is resolved. In short use the above instructions to check you are using the right compatibility mode for the version of Windows you are using and your troubles should be over!!

After I posted above that pdf downloading was working and I didn't know why - it stopped again. I tried your suggestion and changed the compatibility mode and also disabled the protected mode and now it works again. Thanks for the help.