Map zooming in Firefox 3.6.x and probably earlier (phenom X4,, 9500 GT, 4gb ram, XP, very little else running) has been painfully sluggish for me, almost unusable. The zoom animation happens at about 2fps, while years ago it was smooth on much lower end systems than this one. One cpu core is maxed out during animation. It'd probably be much more responsive if there was no animation at all.

IE, Opera, and Chrome are unaffected by the problem. The animation is smooth on them. I sincerely doubt it's an extension or other third party program, and it's not a network problem. Maybe image resizing in Firefox just got really slow lately.