Mozilla Firefox 4 has just been released and I have already given offline installer links for Firefox 4 for Windows, Linux and Mac. PortableApps has also released a portable version of Firefox 4. The portable version of the browser has the same features as the full installable one.
Portable Firefox 4 does not require to be installed and can be run from the USB drive. You’ll need to download Portable Firefox 4 from the below location and then run the installer. It will ask for an installation folder, give the path and Firefox 4 will be installed in the specified folder. You can take that folder anywhere. All the settings including the browsing history, favorites, saved passwords etc will be saved in the same folder. You can copy the folder to a USB drive and then run it anywhere.
You can not simultaneously run Firefox installed in your system and the portable one. One one will run at one time.