No doubt, you must have upgraded to Firefox 3.5 based on my previous review. And now, good things are taken to a next step. Now, Firefox gets portable. You can install it once on your USB pen drive with all those plugins, bookmarks, themes, and take it from one PC to another without losing any of them.
Firefox 3.5 Portable Edition is officially available for download. checkout the detailed Install guide here. Installation and use is easy, requires few user tweaks mentioned in the guide, its not full self-installation.