One of the greatest feature that I love on Firefox is its capabilities to work also on removable media, such as a USB pen for example. I have a 1GB USB pen and on it I've installed a personal version of Firefox and Thunderbird to have always with me when I can't have my personal machine under my finger... 
Today I've discovered that John Haller has packaged both Firefox and Thunderbird into a portable version which can be run directly from a USB flash drive. The projects are called Portable Firefox and Portable Thunderbird respectively and seems very interesting, because they're optimized to work on a USB pen (with selected optimizations to make it perform faster and extend the life of your USB key as well as a specialized launcher that will allow most of your favorite extensions to work as you switch computers).
I've not time to try it now, but I think I'll give it a try soon... 