For web development Firefox is King, or, well, when you do office web development, IE is King. MS uses so many ActiveX components that only they support that all others are unusable.
So I use IE for work and Opera for private browsing. When I take on open source assignments I use Firefox, since Firebug is so d**n good. I agree with Strolen on his assessment of Firebug vs. Opera's tools.
Firefox is much more stable though. I hate using Opera in my online banking, as the JVM support for Opera seems buggy at best.
So, I guess my usage ratio is: