Firefox. I'm still trying to learn programming for the other browsers.
Heck, I'm still trying to find time to learn how to tweak my programs for the other browsers.

IE has so many issues, that I felt it wiser to wait until their latest release was finally done be truly debugged - its still too new.
Safari and Opera are ones I have to deal with next, as Mac users are still at a disadvantage on my site. Linux, I'm too clueless on yet.