Firefox 3 beta goes portable
Because Firefox Portable is self-contained, it will not write any data to your hard drive or registry. That means you can test out Firefox 3's new features like Places, and improved location bar without messing up your current settings. You don't have to install Firefox 3 Portable to a flash drive, you can just as easily install it to a folder on your hard drive.
If you want to run Firefox 3 beta while you have a Firefox 2 window open, you'll need to make one small tweak. Find the FirefoxPortable.ini file in \Other\Sources\ and copy it to the directory that has FirefoxPortable.exe. Edit FirefoxPortable.ini with Notepad, Wordpad, or whatever text editor you prefer, and change AllowMultipleInstances=false so that it says true. Save the file and you can now run Firefox 2 and 3 at the same time.