Some PDFs won't open, when I click on the link I get a blank tab and the screen is black. The same PDFs open fine in other browsers.
I see from your list of plugins that you have the "Adobe PDF Plug-In For Firefox and Netscape "9.5.1" in Windows XP. Could you post a link to a PDF file that won't open in Firefox?
Here is a sample PDF file ... does it open?
If the above file doesn't open either, maybe something here will help:
I also see that you have disabled the Out-of-Process Plugins (OOPP) crash protection feature in Firefox, since your modified preferences show:
I would suggest resetting all of those preferences to re-enable OOPP if you are having plugin problems. Ref:
You have a lot of extensions installed.
Did you check for any problems caused by them?
Start Firefox in Safe Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).