We have Comcast here at home, and Firefox and Safari work just fine on my Mac, and IE works just fine on my parent's business computer and my brother's computer. So I don't think it's Comcast. You probably have a firewall that's blocking IE as a program.
And I didn't realize that a browser could be blocked at that level - I'm aware it happens with certain sites, but I didn't think an ISP would even be capable of doing that.