The Firefox release situation is much more complex than the discussions so far. There are a number of 'release channels' for Firefox, and most of the above refers to the standard release channel. It may be that on some Windows systems the standard channel is set for automatic updates (I don't know) but that would be a function of Windows, not Firefox, and you should be able to change that in Windows.

Normally it's only the Firefox Beta channels that update automatically, but you can change that if you wish (again I don't know about Windows, but it should be possible).

I'm currently running Firefox Nightly 14.0a1, which I updated this morning. I also have Firefox 9.0.1 installed, in case I have a problem with Nightly, which is experimental. (I've had no problems since I started using Nightly some months ago.) BTW, this morning's update took about 5 seconds, including shutting down and restarting Firefox on the same webpage.

There is also a Extended Support channel which gets updated less frequently than standard, and is intended for deployment by system admins with large networks.

Phil