VMware allows you to run one or more virtual machines at the same time on one computer. You can download VMWare Server for free.

The firefox plugin

Running VMWare Server 2ß gives you a firefox plugin that only runs with firefox 3.0 and below. To downgrade firefox, use

rpm -Uvh --oldpackage

Do not forget to downgrade as well Mozilla-firefox as mozilla-xulrunner. If you get the error message

Could not find compatible GRE between version 1.9b5 and 1.9b5.

You need to downgrade mozilla-xulrunner.

TroubleShooting

If you get a pop-up saying "Failed to initialize mouse-keyboard-screen control", you have probably tried to start your virtual machine in an nx session. Better use a vnc session.