I run Vista Business on my Laptop (Have for over a year now on this and previous laptop) and XP Pro SP3 on my desktop. Both run fine and to be honest I prefer Vista now although my Laptop does have 4GB and dual core so the bloat of Vista is not really an issue. Both run business apps, both run as Media Servers for the PS3, both are fine on the wireless network at home or at work, both sync fine with my HTC Touch, I could go on
The only thing I dont like about Vista is the default of not being able to save files in C:Program Files or even modify them. It also took a while before I could get a VPN client that would work but that was sorted shortly after I installed Vista.
As for Firefox, I'd never use IE again if it wasn't for the odd site that needs it for site specific stuff. Although they seem to be getting less and less now.
Just thought of another issue but not relevant for the home user, SQL 2000 is not supported on Vista and cant be installed, I still have plenty of clients that haven't gone to 2005 so it's a bit of a pain if I'm trying to see what they are having problems with and thier on SQL 2000.
Cheers
Steve