Since installing the last round of Windows updates on 3/10/2015 Firefox freezes up my computer and the only thing I can do is to reboot.
I have tried starting Firefox in safe mode and it still locks up. At first I could not access Yahoo mail, now it is starting to happen on other sites with no pattern. It is Windows 8.1 and Firefox 36.0.1.
Start your Computer in safe mode with networking. Then start Firefox. Try Safe web sites.
Do you have any crash reports?
I started having other issues with my computer freezing up so I uninstalled all of the Windows update from 3/10/15 and now I have no problems.