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).