Some Firefox users have reported that Firefox permanently freezes when opening certain sites, particularly Google and Google-associated sites.
I have found one cause of this behavior, and that is if the TEMP/TMP folders of the account in which you're running Firefox is unwritable. In my case, it was unwritable because I was running Firefox in a limited account and had not redirected the TEMP/TMP environment variables to the appropriate folder (away from c:\windows\temp, which was not writable from the limited account). Probably Firefox also will freeze if the disk on which the TEMP/TMP folders reside is full.
You can check the values of the TEMP/TMP environment variables by issuing the "SET" command in a command window that is logged into the appropriate account. You can change the variables by executing sysdm.cpl in that command window, selecting advanced/environment variables, and adding/modifying the TEMP/TMP values in the "user variables" pane. You will need to close all instances of Firefox before doing this, then reopen them afterward. Also, if you start Firefox from a command window, you'll need to close it and reopen it afterward.