Firefox does do automatic updates and has for at least the last 3 years or so. But users '''''do''''' have the option to select "Ask me what I want to do" when an update notification arrives, rather than the update automatically be installed or to disable automatic updates altogether, if they so wish. I hope that isn't going to change, and if it would, I will dump Firefox in a heart-beat just like I did with Chrome over their "updater" process running all the time. The "trigger" for an automatic update hasn't always been enabled on the first day an updated version is released, but seems to have been turned on within the first couple of weeks. At least that was the case when new versions were being released at 10 to 14 month intervals, but with the new Fast Release program, it is going to be turned immediately - as was done for Firefox 4.0.1 users. Firefox 3.6.17 users got the 3.6.18 update initially, but a week or so later they started getting a notification about the Firefox 5.0 upgrade. As far as backward/forward compatibility, forward compatibility is tested, but backward compatibility isn't been tested, AFAIK. Most users going back one major version don't have problems, with the exception of when users were going back to Firefox 2.0.0.x when Firefox 3.0 was released slightly over 3 years ago. Users who went back to Firefox 2 didn't have access to any new bookmarks saved while they were using Firefox 3.0, due to major changes in the way bookmarks were stored. Then when they returned to using a 3+ version, whatever bookmarks they saved when "back in" Firefox 2 were gone. The "fix" was simple, if the user took the time to search the various Firefox support pages for the answer or asked us how to cope with the change.