Could you do the following in safe mode and then boot to normal windows and let me know how it behaves. Also what error do you get when you try to run Firefox ?
Next we will check for driver conflicts
Step 1: Start MSConfig
Click Start, type msconfig in the Start Search box, and then press ENTER.
If you are prompted for an administrator password or for a confirmation, type the password, or provide confirmation.
Step 2: Configure Selective Startup options
1.In the System Configuration Utility dialog box, click Selective Startup on the General tab.
https://dl.dropbox.com/u/73555776/Cleanboot1.JPG
2.Click to clear the Load Startup Items check box.
Note The Use Original Boot.ini check box is unavailable.
3.Click the Services tab.
https://dl.dropbox.com/u/73555776/cleanboot2.JPG
4.Click to select the Hide All Microsoft Services check box.
5.Click Disable All, and then click OK.
6. When you are prompted, click Restart.
Once back in windows does the problem still occur ?