One cause of Firefox 2.x suddenly not loading is corruption in the Firefox section of a user profile.  This can be verified by logging into the affected system with a different user account.  If Firefox works – then the problem is in the profile.  The common solution is to delete the folder, but that will delete all custom settings, including bookmarks.  One file, in particular, can become corrupted if the browser crashes, and renaming or deleting only that file may fix the problem in a less drastic way.  It is “xpti.dat”, and is found in the Firefox section of the user profile, which is normally found here:

C:\Documents and Settings\username\Application Data\Mozilla\Firefox\Profiles\randomstring.default