I was going to say GMail but I can't say it's BAD overall (in fact it's way better than Hotmail). However recently they made a few changes which were all just terrible. The Compose Mail link was a bit too anonymous before, but that big ugly button they've got now?? They also did something to the menus on the left which just made them weird.

Not to mention the fact that the button to delete an entire "conversation" is relatively big, and right at the top of the working area, while the control for deleting one message in a conversation-- a far more common operation-- is buried in a pull-down menu.

I'm sure the keyboard shortcuts would be awesome if I'd ever had a browser / OS combination that they worked on.

What doesn't work? FF + Windows has always worked for me.