Anecdotally, I have an oddly large population of vistors to my site with Firefox 9. I don't know what changes were made to how Firefox updates around that time. But it seems like the update train has unwittingly left few users behind.

As a reference, Firefox 9 is the version in the packages repo for FreeBSD 9.0-STABLE and it's probably the same for a fair number of other oses that peg versions on release.

That is correct, only a few percent of active Firefox installations still run 3.6, and the majority of those are active less than once per week.

There are a few dozen support staff employees at my company still using Firefox 3.6, as that is all the vendor of our product supports. Luckily, they are on a local network, with no Internet access from these machines.