Firefox 3.6.15, mostly because I don't want to go through the process of figuring out which add-ons are now broken and how to fix or replace them like I have to do with every major release of Firefox. Chrome never interested me, and I really don't want to spend a bunch of time looking for add-ons that may not exist. I'm down to 59 extensions (this will likely go down again since I'll prune in process of moving to Fx 4), and replacing them is a daunting task I'd rather not take when there's really no compelling reason to switch to Chrome (except the tabs as processes feature, but sessions are saved automatically by Tab Mix Plus). BTW, if you think 59 is a lot, I was up to around 100 when the latest Firefox was 3.0.x. Over time, extensions can break (either on their own or with a major change to Gecko), are unneeded, or are unnecessary when their functionality is built into Firefox, so that number will steadily decrease.