As written in Mozilla Firefox 3.1 Beta 2 homepage
Install Note: The 3.1 Beta 2 release will install alongside a Mozilla Firefox, Portable Edition installation by default and show up in your PortableApps.com Menu with a "Test" suffix. The installer supports upgrading existing 3.0 installations by manually changing the install location as well.
But the "Test" suffix didn't shown in the PortableApps.com Menu ..Just show Mozilla Firefox, Portable Edition ...
I need to put the mouse on the menu and see the folder location to identity which one is 3.1 and 3.0
Hope you can revise the problem soon.
That was an oversight on my part. Sorry about that. It'll be rectified soon, though.
Sometimes, the impossible can become possible, if you're awesome!