Yes, yes, we all know that Firefox under wine beats Linux Firefox for javascript performance. That's not what I'm talking about though.
I'm talking about the UI performance. Things like switching tabs and scrolling. To me, the Linux version of Firefox in Intrepid or Jaunty was always inexplicably more sluggish compared to the Windows version.
I think I figured out why.
The fix is simple. Stop using the Human GTK theme. Use one of the plain ones, like Simple (System > Preferences > Appearance > Theme > Customize > Controls). As soon as I made the switch, everything got much snappier. My guess is that Human makes firefox do some extra drawing, and that drawing is on the wrong path.