Yes, the scroll increment seems to be smaller when using the mouse wheel in FF as compared to IE (I'm using an Intellimouse optical as well, just with the default WinXP drivers). Now that you've pointed it out and I've compared the two browsers in this rgard, I do wish the scroll wheel moved the page along a little faster in FF, like it does in IE. What would be even nicer is if they added a slider that allows each user to set the scroll rate for the mouse wheel that they are most comfortable with.
But I wouldn't really describe this as "smoothness" myself ... it's just a question of how much a person has to spin that wheel to move to the part of the page they want to see. And there are so many other things about FF that are markedly superior to IE that I'd personally never think about giving up FF only due to something like this.
edit: Oh, and by the way, I don't want to change the mouse wheel speed setting in the WinXP Control Panel, because then all of my other wheel-using apps feel out of whack. FF seems to interpret the height of a single "line" somewhat differently from the other browsers and programs.