After giving Firefox the OK to update some add-ons, I restarted the browser and it froze as soon as it tried to open the page tabs I had open before restarting. So, what do I do know, if I can't close those crashing tabs but only close the crashed Firefox? Well, I peered into my Firefox profile file list and found sessionstore.js, the file that stores the data about the session and tabs which must be restored after the browser is restarted. Just delete that file and launch Firefox again.
 C:\Documents and Settings\username\Datos de programa\Mozilla\Firefox\Profiles\profile-code-string