Its the Firefox web browser that can be ran off your USB. Which has a few purposes, for example if the machine your on restrics access to a browser.

Mozilla Firefox Portable Edition is a fully functional package of Firefox optimized for use on a USB key drive. It has some specially-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. It will also work from a CDRW drive (in packet mode), ZIP drives, external hard drives, some MP3 players, flash RAM cards and more.

DOWNLOAD:
http://nchc.dl.sourceforge.net/sourceforge/portableapps/Firefox_Portable_3.0.1_en-us.paf.exe