"If you've installed version 1.5 or 3.6, your installation will be upgraded to the current one (I think I saw ver 12 or 13 is current now?), if the article means what it says." - John
It may well mean what it says, but it is from an uneducated source, and in this case is wrong. :)
There may be a lot of MS-Windows users here where that could be true in some instances because of configuration choices. I am not very educated on Microsoft operating systems. However, the machine that I installed 1.5 on did not have the resources to run even 3.6 so an "upgrade" cannot be done without rendering the machine useless. Also, I run FF 3.6 on this very capable machine here and it will not be changed without me doing it. The reason is that another user (Mozilla) does not know my password and I don't run as root. We run *NIX systems here (lots of them) but I'd be surprised if any of the Windows OSs could not be made secure as well - at least to the point of preventing outsiders from installing software without the user's consent. Simply put, that would just be inviting malicious employment.
Apart from hardware capabilities, I think the main choice of version would be dictated by whether or not you need Adobe Flash for things like YouTube. Of course, there are dozens of other browsers one could chose. I have FF, Dillo, Chrome, Epiphany, and Galeon running 24/7 on different desktops for different purposes and if one was to lose my favour it wouldn't be a big deal to just blip it. Anyway, all this talk reminds me of why one might want to run Opera. They don't espouse the same shenanigans that FF seems to enjoy.