Just to clear some things up (note I don't work here).

They said that the freebies are hosted by Amazon's EC2 servers, so that by default makes it a distrubuted network.  You are likely getting better speeds because in all honesty the number of 'hops' the route between you and the EC2 server you are near is likely to be less then the number of 'hops' between you and whereever Renderosity's store is located.

The internet as a whole is very dependable, however the given route between any two points is much less so.   It is fairly common to test different points (in this case ISPs) to determine exactly where the issue lays. Often times however, the issue is in fact, Comcast.

This is clearly a networking problem, and as such switching browsers isn't apt to have any effect in the speed the server sends data.  Pulling the following product page at random:

http://www.renderosity.com/mod/bcs/calendar-girl/99740/

The client is making 166 requests, at a total of 3.2 megs to render that page.  In this case the sheer number of requets is really hampering the page load, as different clients make different amounts of HTTP requests at the same time.

Running Page Speed on that page, generates a score of 71/100, however flags server response time as an issue to look into.   In short, it took the server almost half a second to start return the HTML to my client to even begin the parsing, then another 300 ms to return the file in full to me. (So by the time the HTML has been returned, 3/4ths of a second have passed).

This really points to the webserver being slow or under load.  I don't know the internal configuring of Renderosity's network, but I would personally in this situation really be looking at the server load, the load balancer (Because I am assuming there is more than one webserver), and the Firewall configuration.

If internally they are not seeing issues, I'd have to really question if they are accessing the site 'externally' from an internal location or not.  If interally they are taking a different route avoiding the Firewall's external config, for example, they are very apt to get different results.

Some fun details the support staff may like:

On Roadrunner in the North East, pinging renderosity.com never returns a value, the server always times out.  I get the same results when trying to ping the domain from network-tools.com

Running Trace Route from multiple points I always start getting time outs once I enter the following domain structure:

*.nd1.nas.peak10.net 

So whatever the next hop from there is, is likely to be part of your problem.