When using FF to access facebook, firefox locks up. The only way to to restore is to use the ctrl+alt+del to shut down firefox.
Start Firefox in Troubleshoot Firefox issues using Safe Mode to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).
If it does work properly in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.
See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems