Firefox vs IE configuration problem?

I just installed Windows 2000 on a PII 266 box (new hard drive) which
formerly had Windows 98 (old hard drive).  The hardware and software
(specifically, Netscape) all worked when the system was Windows 98.

I can now run IE and Outlook, no problem.  Firefox, Thunderbird, and
Netscape - no.  To attempt to run these programs causes the modem to
disconnect.

Does anyone know why the system might act like that?  The modem is a (no
name) with a Conexant HSF chipset.  Microsoft had a signed driver which
didn't make any difference.  The manufacturer (Sterling Communications) had
a driver which didn't help.  Conexant's 'stop gap' driver - same.

Is this a configuration issue?  What setting might cause this behavior and
can it be changed?

Inquiring minds want to know.

Thanks,

QuasiPages