Comcast and most large ISPs only support IE6&7 because they havent been trained on any other web browser. The only drawbacks with Opera, and Firefox etc is that they handle some HTML scripts slightly differently than IE. This can cause some pages and links to behave incorrectly, and cause problems.
It's prety rare though, as the latest releases of FF and Opera are almost fully compliant. (some of you may remember how netscape navigator couldn't render some web pages corectly)
As far as speeds go, use what you like best, as long as you have set up the cache, and refresh options correctly (defaults are usually fine), the speeds should be almost identical.

/Network engineer for a large ISP