I’m still using IE6 and I also use FireFox. When I went from XP Pro SP2 to XP Pro SP3, I really didn’t notice any difference. There were no advantages except the obvious…to stay current with MS…and frankly, there were no disadvantages, no problems at all.
All in all, pretty innocuous.
A word of caution. If you’re using IE6 and you “upgrade” to IE7 prior to installing SP3, you can’t go back…at least not without unistalling SP3.
If you install IE7 after the upgrade to SP3, you can revert to IE6 no problem.