I had a Dell Latitude E6220 on 32 bit Windows 7 SP 1 and each time I'd open up Firefox in any profile, the screen was totally black. This did not happen in safe mode (hold down shift while starting Firefox) or when I RDP'd to the computer. I reset Firefox settings to no avail.
The solution was to disable hardware acceleration via Options -> Advanced. If your Firefox is opening up with the black screen, you can disable hardware acceleration in safe mode.