I'm using Firefox. I've used it since the early betas pre-1.0 and I've just gotten used to it. I've used Opera and a multitude of other browsers, like some Linux browsers, but I find myself going back to Firefox every time.
I'm using IE7, sometimes I need to use Firefox too thought, simply because sometimes I get images that are ready and able to be displayed on Firefox. Other than that reason, I am a loyal Internet Explorer user, the easiest browser.
Web development and cross web browser compatibility is a very serious issue that should be addressed by any professional web development company or individual web developer. Poor web browser compatibility could mean lost sales and customer enquiries!