I looked at reports for three sites for which I have analytics access.
Here're the results. IE: 53%, 58%, 71%, Firefox: 29%, 27%, 19%, Safari: 13%, 9%, 4%
One of the sites is for commercial vehicles. One is for motorcycle gear. The other is an alternative wedding site.
I see a lot of likely demographic differences. All of this is supposition, with no studies to back it up, so take it with a small mountain of rock salt.
There's likely an age difference among those sites. I'd guess that two of the sites skew older than the other. There is also a likely difference in income: one being primarily people with disposable income, one for people with less disposable income, and one being primarily small to medium business owners. There's likely a gender gap among visitors to these sites: one of the sites is probably 75% women, the other two sites probably have 85+% men.
Given those assumptions, I'd argue that young, affluent women are more likely to upgrade their browsers than older small-business owners.
In other news, I'm beginning to think water tends to be wet at room temperature, and ice is not very warm relative to fresh waffles.