You can put a Linux distribution on a USB flash drive and boot that,
and if you like it you can install it. http://www.browserlinux.com/
is a small lightweight one, maybe it would be a good base for a Webian
LiveCD; Google "lightweight linux distribution Firefox" for many
I think BrowserLinux ships Firefox 4.0.1 right now. The instructions
for BrowserLinux are very bare-bones, I wouldn't recommend it unless
you've made a bootable USB flash drive before.