I've had a couple of reports of PDFs freezing in Firefox, by either not loading or locking up as you scroll pages.
It seems to have just started so is probably part of the latest release which has automatically upgraded recently.
So far I've found two things to try that seem to work. One is to kick up the Offline Storage setting to 100mb as per the instructions put together by Jason at 404 Tech Support. Click on the image below to see how to change this setting:
The other thing that has worked a couple of times is to go to Tools - Add-ons - Plugins and disable previous and alternative versions of Adobe Reader plugins.
The GATCF PCs are unaffected (using Foxit).
On a vaguely related note - don't upgrade to Firefox 4 if you use Webreporter - it doesn't work according to HORIZON-L