I also have this problem of Firefox updating by itself even when I turn off auto-update. For whatever reason, there are situations where you do not want to update or download a large file, because you're working offline, have a slow connection, or have a very low download allowance etc. Usually the update doesn't work and cause Firefox to crash. After that it cannot be started again. This is with Firefox Portable but I think I've had this problem the normal Firefox. With the portable version I can just back up a single folder but it is still time wasting and annoying. I've noticed that when I start Firefox with auto-update turned off, it changes the options to auto-update, it also does this periodically. So every time I start Firefox I have to turn off auto-update, and I have to do this from to time because Firefox will by itself turn back on the auto-update.