As usual, Firefox asked me to upgrade... but it upgraded to 3.5.10, and then it has a link to see the "latest version", which is 3.6.6
So, a bit strange, why Firefox doesn't directly upgrade to 3.6.6?
Here's my thought: the difference between Firefox 3.5.9 and 3.5.10 is mostly in the form of bug fixes. There might be some very minor changes in the way the browser looks and operates, but usually when you only change the third component of the version number, the program stays mostly the same. So it's very unlikely that someone would have a reason to not want to make that upgrade.
However, if you go from 3.5 to 3.6, there are quite a few new features and significant changes to the appearance and function of the program, which could affect how you use it. For example, some of those changes could break compatibility with add-ons you might have installed. In that case, the Mozilla people probably want you to examine the changes and think about whether you want to make that upgrade or not.
There might be a technical reason that they're not able to upgrade 3.5 to 3.6 automatically, but if so, I don't know what it might be.
maybe the upgrade can only go from your specific version to that specific version. once its up to that level, it can then be upgraded further