14 DAY TRIAL // Firefox 3.6 users will be upgraded to Firefox 12, the latest stable release, in early May, whether they like it or not. The ancient Firefox 3.6's end of life was on April 24. No more security updates will be issued, leaving users completely defenseless.
There may be millions of Firefox 3.6 users left, Mozilla can't know for sure. Those that have automatic updates enabled will have Firefox 12 installed on their computers. The only way to prevent this is to disable automatic updates.
Given that there will be no more minor updates for Firefox 3.6, users can do that with no side effects, as long as they realize that they'll be vulnerable to all sorts of attacks.
The move is less controversial than it seems, Mozilla is forced to either leave users to their own devices and be responsible for millions of infected computers, or force them to upgrade.
Neither choice is particularly appealing, but Mozilla would rather have users that are angry but safe. At this point, most remaining Firefox 3.6 users are either skill-less with a computer or simply want to stick to their old browser because they got used to it.
PowerPC Macs will be abandoned
However, there are several particular cases where the upgrade is not as straightforward. On the one hand, users still stuck with PowerPC Macs won't be able to upgrade to a newer Firefox at all. But they've been long abandoned by Apple so Mozilla can't really do anything about it.
Windows 2000 and XP (pre-SP2) users will be stuck with Firefox 12
Then there are Windows 2000 and old Windows XP (pre-SP2) users who can upgrade to Firefox 12 but won't be able to upgrade to anything beyond that. With Firefox 13 Mozilla switched to Visual Studio 2010 which no longer supports the old versions of Windows. Those have been abandoned by Microsoft as well.
XP users can install the latest service pack, Windows 2000 users will have to upgrade their operating system or switch browsers, Opera being the only viable option left.
Firefox 3.6 users running Windows 2000 or old XP will be upgraded to Firefox 12 even after Firefox 13 is released. Users won't be upgraded to Firefox 10 ESR
There will be no automated upgrade path to Firefox 10 ESR (extended support release) since it is designed for enterprises and companies. Regular users are not supposed to be running an ESR build and won't get any sort of support they'd expect in a regular Firefox release.