It seems to be a bug with Mozilla that is causing screens to freeze.
To unfreeze your screen:
1. Make note of the name of the machine you are on.
2. Log into another machine
3. ssh into the machine that is frozen
pkill -9 -u your_username mozilla
br> 5. This should kill the mozilla process that is running and your screen should return to normal.
If not, type
pkill -9 -u your_username metacity
Your window manager should restart and your screen should return to normal.
To avoid Mozilla from freezing up when viewing pdfs:
Save the pdf to your home directory.
This is the only thing that can be done until the next release of Mozilla comes out.