Do you have any PDF reader on your computer? If not, that might be the computer. Make sure Firefox has a plug-in which allows it to actually understand a PDF format file.

Try installing the latest Adobe Reader from here: http://get.adobe.com/uk/reader/

Make sure to untick Chrome from the donwload, if you don't want it, or have it already on your system.

Make sure you have a local installation of a PDF reader available. I recommend against using the Adobe software and going to oldversion.com to download an earlier version of Foxit reader (I recommend with going something before the 3.4 version to avoid the annoying Ask.com adware that's associated with newer versions of Foxit. After you've installed Foxit reader, download a PDF file and save it to your downloads folder, open up Windows explorer, right click on the pdf file, select "Open With", click the box that says "always open files of this type with this program", then select Foxit Reader. That should solve your problem.

Thanks for the replies, this is NOT a PDF reader issue. I have the PDF opened in a separate program, not Firefox. This is a problem with Firefox being the default browser and since the update Windows can't seem to find Firefox or access it when I am clicking from a link in say a Word Doc or PDF or Chat Window. I've had to resort to using Chrome as my default browser and everything opens just fine with that.

Seems like switching to Chrome as the default and then switching back to Firefox as default has fixed the problem. Sucks to have to do that after an update.