This is an unofficial build of Firefox browser (the official build of Mozilla Firefox modified and bundles with a PortableApps launcher, you can take your bookmarks, extensions and saved passwords with you) that is designed to run from removable devices such as USB sticks, CDRW, MP3 players, etc.
To install Firefox Portable, just download the portable package from Portable app.com, and then double-click it. Select the location (USB device) you wish to install to and click OK. It will create a Firefox Portable directory on your drive and all the necessary files. Click on the Firefox Portable.exe file from this directory to run the program. Make sure to close locally installed Firefox browser before running the portable Firefox.