One of the requests that I frequently receive with every Firefox 3 Alpha release is to put together a portable build that people can play with. That way they don’t need to worry about messing up their existing Firefox installation or profile, and when they have had enough with it they can simply delete the folder it resides in.
Download Squad found a site that ingeniously created a script which automatically downloads the latest nightly release of Firefox. That means right now it is grabbing the nightly builds of Firefox 3, and after that branches off it will begin grabbing Firefox 4 nightly releases.
All you have to do is download the version of Portable Firefox that includes the updater (Download Mirror). After you extract the files you’ll see a file called Updater.exe, and that’s what you need to run first. The download of the latest Firefox 3 nightly build will be initiated, and you should see the download progress in the System Tray (similar to the screenshot I took above). After the download is done the Updater will extract the contents using 7-Zip (already included), and it will place the files in the appropriate directory.
How long the actual process takes depends on how fast your Internet connection is, and for me it was done in less than a minute. Overall this is a great idea, and I think it would be even more awesome if there was an option to specify the URL of the download. That way you could pick and choose which version you wanted to run!
Instructions on doing this process manually can be found here, which also includes details on copying your existing Firefox profile over to the portable setup.