Firefox problems on Linux domain

I am a severely underqualified administrator of the web and file server at the school where I teach.   Today's problem has me stumped.  

I just added 3 users to the server and to Samba.  They do not have access to Firefox.  When they try to open Firefox, it tells them the proxy server is not found.  The details:

1.   Two additions were new teachers; one is a student.  They were added to the appropriate groups.
2.  Workstations run Win2000.
3.  Server is SuSE Linux 9.1.  Not sure what version of Samba.
4.  Previous to adding these 3 users, everyone could use Firefox.  
5.  Internet Explorer works fine for everyone.
6.  I can ping the server; the proxy server (Squid) is running.
7.  I can use Firefox (I am an admin); students cannot.  Not sure about other teachers.
8.  Network was setup by a programmer who also didn't know what he was doing.  There is no documentation.  It was set up last August.  I do not have access to this person to ask questions.

Is there something in the Samba config that I should look at?

Help!!

TIA, Kari Matthews