Actually, Netscape 6 and 7 are based on Mozilla not the other way around. So if they say it will work with Netscape 6 and 7, then it will work with Mozilla. Orginally Mozilla was to be based on Netscape 5 code, but the Mozilla developers decided it was a lost cause to try and fix it, so they tossed it out after a year of development and started from scratch (which if Microsoft hadn't sat on the browser market suffocating Netscape out of business, Netscape planned to do anyway after 5.0).
Netscape 6 is based on code a few weeks prior to the Mozilla 1.0 release, Netscape 7 IIRC is based on something around Mozilla 1.4 code.
The instructions for setting up things with Netscape 6 and 7 should apply to Mozilla as well. If that doesn't work, lie and say you're using Netscape 7 or something. A lot of people in phone tech support are what you call punters. They're going to be looking for ANY excuse they can to blame something else for the problem so that you hang up the phone and they can get to the next person. You have to be clever enough to translate any differences in instructions in real-time.