Portable Firefox
Portable Firefox is useful if you work from one place to another and not each computers have Firefox installed. It is a fully functional package of Firefox and can be run from USB Key drive, Zip drives, External Hard drives, Mp3 players such as iPod shuffle, etc.
There is also Portable Thunderbird, Portable Sunbird, Portable NVU and Portable OpenOffice.
Unrelated to Mozilla Firefox, if you use a lot of portable software there is also TiddlyWiki.
TiddlyWiki, an experimental MicroContent WikiWikiWeb built by Jeremy Ruston. It’s written in HTML, CSS and JavaScript to run on any modern browser without needing any ServerSide logic. It allows anyone to create personal SelfContained hypertext documents that can be posted to any web server, sent by email or kept on a USB thumb drive.