I want to use Linux fully. But I'm using both RedHat 9 and Windows XP,
because of one problem. Please help me to use Linux only. My Problem is:

My Father is running communication. He is using netphone for international
calls, means VoIP. And for that we are using account of quantum communication To access account we have to Login
qcnltd.borderproxy.com
Now my problem begins. I can't give you the login information but
after logging in there is a menu bar. The menu is drop-down menu.
Internet Explorer easly slides the menu, and I can acces it.
But mozilla is unable to drop the menu down.
I've tried several other browsers but same problem.
I am able to log in but unable to access functions like recharging account, balance transfer, etc..

How to enable that drop down menu in mozilla browser so that I can
say that I'm a Linux user.

Site menu could be a possible flash based menu system. May want to see if that is true. also Don't think it is a javascript problem as you mentioned trying multiple Browsers.

You really need to update to a more current version of Linux. Support for RH9 was dropped in 2004. After RH9 they started numbering again using RHEL(Red Hat Enterprise Linux) and they are currently on RHEL5. Since RHEL is not free you may want to consider Centos. Centos is RHEL with the logos removed. It is binary compatible with RHEL and is free to download/update. Fedora (F10 being current) is also another choice but only has support for 13 months (RHEL is supported for 5 years plus).

Pretty much the only supported (by most websites) linux browser is going to be FireFox. For those rare websites that one must use that require IE have a look at IEs4linux(just google it).