On Twitter, Kay Steiger pointed me to this data on browser shares over time. I made a chart taking the May data of each year.
I'm a Chrome user, except for the fact that the Slate CMS only really works properly with Firefox so I usually keep a few Firefox tabs open for that. I'm always checking back with Safari since it sort of seems to me that Apple should be in a position to make the best Mac browser, but so far they haven't convinced me. Maybe Mountain Lion will change things.