Step one get the OS X version of CrossOver and the windows installer for FireFox, look for the little link to 'other systems' under the big FireFox button.
Save this file to disc and install CrossOver and run it.
Choose install software and from a file, browse and open the firefox file.

When it has installed and you have FireFox open go to the www.quakelive.com website. Here you will be prompted to install Flash.

You will probably have to do the by clicking on the manual install. This will download in the Windows FireFox download manager. Wait for it to download and then double click on the file in the download manager.

This will say you need to close FireFox. So close all the Firefox windows and leave the Flash installer open. When FireFox is closed click try again.

The bottle will reboot after installing Flash and show you a Finder window with FireFox. Double click on the FireFox icon and it will launch the Windows version with Flash installed.

Now you will be able to log into the quakelive site like a windows user. As I'm now only number 13,000+ in the line then it might take some time to see if the plugin works...