This is simple but brilliant! Alex made what he called "FoxGlove", it is a portable Firefox 3.0 based, fully customized to better integrate with all Google Apps such as Gmail, GCal, Docs, GTalk, Notebook and so on. It is very easy to take on a USB key everywhere. It also integrate a Google Chrome-like theme and you can have several versions with different Google Accounts open at the same time. Indeed it is very very usefull...at least for me ;-)
Why Google did not think about this before?
Defenitively there is no limit to Firefox customization power!