I use Firefox and already tried Chrome, Opera and Safari.
I still like the classic browser look of FF.
I enjoyed a lot the excelent multitasking of Chrome and its ability to resist to plugin crashing.
Auto-hiding the toolbars would be a great improvement.
Maybe some sidebar with autohide, customized with some mini-sites or application plugins like clock, weather, notepad, some site, etc.
Use the lower part of the browser window to have some useful custom information like time, quotes, weather, news, etc.