(Access the raw data and a larger graph at Google Docs.)
It turns out that Firefox 3.5 is even less used than 3.0, both well under 5%. On the other hand, as mentioned, 3.6 is still 25% of usage. It may well mean that we drop 3.5 support and take advantage of some of the behind-the-scenes things available only in 3.6 and higher very soon.
P.S. There's also some graphs, by major version, and minor version within 0.9, of Greasemonkey usage.
Nerd alert: To make this graph, I started with the AMO statistics, and passed it through a python script to clean up and combine the (patch level) versions. By hand, I rearranged the data into a 7-day moving average for the versions I was interested in: 3.0 and higher.