Today Firefox is the most using browser.. because not only windows but linux users are also using it.. If u r asking about better browser so Internet Explere have better display of HTML tags then firefox.. To display better in firefox we need use as much as property we can.. IE mostly ignore required properties and display better view automatically.
I have seen many of the site's statistics and those statistics showed me that the most usable browser ( currently ) is Opera, I don't know why but think that on 2nd position is Internet Explorer as its much easier to use, and most of the softwares do install the plugins for IE as compare to opera or firefox. Like Flashget download accelerator, when installed it do provide a plugin for IE but in opera you can't use such Flashget shortcuts, also some of the softwares for filling the forms, etc. do help with their plugins for IE.