First things first, follow this link and grab yourself a copy of the installation file and run it. Given it's still in beta, the installation will prompt you to install to a different directory to the standard Firefox installation, so you can uninstall the beta without messing up your previous installation if you have any problems.
When you start the browser for the first time the new add-on manager will check the compatibility of your existing add-ons and enable them accordingly. Unlike previous versions of Firefox, uncertified add-ons are disabled so even if your old add-ons could work, you won't be able to use them until the Mozilla team have verified them, which was the case with the majority of my add-ons.
This may sound like a big inconvenience but it's actually all a big part of Mozilla's push to make the Firefox platform a stable and secure one. Indeed one of the major enhancements to this latest version has been the plugging of various memory leaks that previously saw Firefox using in excess of 1GB of memory when used for extended periods. Now the majority of these problems have been fixed and Firefox 3 is faster to load, more responsive, and much less prone to grinding to a halt. By ensuring all add-ons are secure and stable the Mozilla team can be certain that all their hard work isn't then put to waste be a badly coded add-on.
Once you've passed the add-on menu you'll be greeted by the all too familiar Firefox environment and you may be hard pushed to notice the difference except for the Firefox 3 beta welcome page - how awesome!? Indeed, if you've installed the beta with another version of Firefox already installed, all your bookmarks, homepages, and many other settings will have been added, so everything should be just as you left it, so you can get to work straight away.
Aside from the performance enhancements, a number of other headline features have been added to this latest version. The first of these is a brand new version of the webpage rendering engine, dubbed Gecko, that completely revamps the way Firefox actually handles and displays web content. Most of this is complicated background stuff aimed largely at improving performance and compatibility - something which is evident when you see how much faster pages load - but there is one obvious and standout addition, as well.
Gecko now enables the user to scale whole webpages, just like in IE7, by holding down Ctrl and rolling the mouse scroll wheel. In previous versions only the text was scaled so web page formatting regularly fell apart. With the new version the whole page can now be shrunk, including images, embedded content (i.e. flash and video), and forms. The result is a much more flexible browser that is ideal for use on notebooks or desktops where screen size is an issue. It isn't perfect and every now and then the page will break slightly but generally if you shrink or expand the page one notch further it will right itself. Also, pages can't be shrunk quite so far as in IE7 so you'll never be able to see the whole of the TR front page on one screen, for example.