If you plan to access the Web from public computers or from work, security and privacy should be your primary concerns. While Firefox allows you to remove all the private data saved on the disk, it does nothing to secure the Internet connection itself.
For security and anonymity, you can use Torpark, a combination of Portable Firefox and Tor. Tor uses a distributed network of servers called onion routers to create a secure and anonymous connection to the Internet. Torpark comes with integrated and preconfigured Tor software -- just launch the application and it takes care of the rest.
Torpark is rather slow, however, so if you want to use an application that demands a low-latency connection, such as the OpenWengo VoIP application (which I'll talk about in a moment), then you would be better off using it with Portable Firefox