Comcast's ASNs and networks are documented in ARIN's WHOIS database and various route registries. Hell, Comcast probably publishes a list on their own web site.
So, yeah, Mozilla can easily determine if a user is on the Comcast network just from their IP address.
Also, while Comcast actually has a bunch of DNS servers spread across the country, I believe that nowadays they're mostly "promoting" the use of 75.75.75.75 and 75.75.76.76 (which, AFAIK, are anycasted and direct end users to their "local" DNS servers).
> So, yeah, Mozilla can easily determine if a user is on the Comcast network just from their IP address.
I mean, how can the determine it's a Comcast DNS server configured on my network? I might be a Comcast customer w/ a custom DNS server configured. If it's a fixed IP check, I suppose that list is in the browser.
What makes you think they are doing that? To jlgaddis's point, they are likely checking what IP address you are coming from over the public Internet. They can see who operates it, and knows that it's Comcast. Then they change the options in your browser.
