By default Firefox is set to automatically update. That can be changed in Options > Advanced --> Update

Thanks you for the quick response. Seems to me, the default should be to notify. And then allow the user to change the options. And there does not seem to be a way to back out of the update once installed.

6.0.1 is an important security update that deals with "missing" security certificates from one vendor, and another is on the way to fix "the fix" (Mozilla blocked too many certificates), hopefully later today or tomorrow.

Once you have restarted Firefox and the update is actually installed, there is no way to go back. You could have closed Firefox and cleared the update file from the bowels of Firefox and stayed with the previous version, but once it is installed you can't go back.