All of them... I'm a web developer... When I'm just browsing, I just use whatever browser is already open, otherwise, I tend toward IE9 or chrome, just because I like the HTML5 stuff... it's new and shiny.
I usually use Chrome but I am playing a game where I kinda ehhh... have several accounts and firefox has a really cool feature to make profiles and I can play from different accounts at the same time. So I sometimes use firefox but mostly Chrome
I use firefox. I tried swapping to Chrome, but when I found that, in addition to not having an ftp addon, it had issues with PhpMyAdmin as well, plus various other little problems, I soon went back to the fox. The latest version seems to have speeded up a bit, which is welcome, as it had been getting pretty sluggish.
I can't use IE (and wouldn't if I could), because I decided to give up viruses and swap to Linux several years ago. It's faster, has more capability and it's free.