While you can connect to the administration Web site of a Virtual Server host using Firefox, you will not be able to start a virtual machine or access the embedded Virtual Machine Remote Control client.
If you're like me and enjoy the features of Firefox, you probably also find it annoying to have to navigate between two Web browsers to access different sites. I'm not quite sure when I actually reached the point of laziness where opening up another application became too much for me, but for some reason it is. Anyway, let me get back on track.
The simplest solution for you in this case is to download the IE Tab extension for Firefox. With this extension installed, you can run IE within a tab in Firefox. The only catch is that you need to specify which sites should use IE. All other sites will default to using Firefox. After you install the IE Tab extension, you will need to close and re-open Firefox for the extension to be available. Once you open Firefox, click the Tools menu and select IE Tab Options. Under the Sites Filter tab in the IE Tab Options dialog box, just enter the URL of your Virtual Server Host system (example: http://vs1.mcpmag.com). When you load the Virtual Server Administration Web page, you'll see an IE icon in the address bar, alerting you that the page is loading under IE.
If you're just looking to connect to and manage individual virtual machines, you can simply copy the VMRC.exe file from the Virtual Server host to your system. This file can be found in the VMRC Client folder within the Virtual Server installation directory (C:\Program Files\Microsoft Virtual Server\VMRC Client, by default). By using VMRC.exe, you can connect to and run any virtual machine without needing to connect first with a web browser.
Finally, one other management tool that you may want to check out is VSPlus. VSPlus is a free, stand-alone Virtual Server management tool. When run in my test lab, I received a few error messages while running the tool, but I did not have any problems using it for basic server monitoring and administration, as well as connecting to and managing virtual machines.