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The last beta release for the long awaited Firefox 3 is now available for download, featuring a whooping 750 changes from the previous F3b4. The warning as usual is for testing purpose only, and installing on a work critical computer is highly not recommended.

The benefits for early testers will be speedier Javascript handling in webapps like Gmail and Zoho Office, better Windows/Mac/Linux integration, and an improved Places (bookmarks) organizer. After the jump, see the list of Beta 5 improvements pulled from the release notes.

• Integration with Windows: Firefox now has improved Windows icons, and uses native user interface widgets in the browser and in web forms.
• Integration with the Mac: the new Firefox theme makes toolbars, icons, and other user interface elements look like a native OS X application. Firefox also uses OS X widgets and supports Growl for notifications of completed downloads and available updates. A combined back and forward control make it even easier to move between web pages.
• Integration with Linux: Firefox’s default icons, buttons, and menu styles now use the native GTK theme.
• Places Organizer: view, organize and search through all of your bookmarks, tags, and browsing history with multiple views and smart folders to store your frequent searches. Create and restore full backups whenever you want.
• Speed: improvements to our JavaScript engine as well as profile guided optimizations have resulted in continued improvements in performance. Compared to Firefox 2, web applications like Google Mail and Zoho Office run twice as fast in Firefox 3 Beta 5, and the popular SunSpider test from Apple shows improvements over previous releases.

If you don’t feel like braving the beta, you’d have to wait till June for the final release. In the mean time, if you decided to dive in there’s a very simple way to enable your incompatible extensions.

Download the Nightly Tester Tools extension. Once it’s installed and you’ve restarted, your Firefox 3 Add-ons dialog will have a “Make all compatible” button. Press that puppy to make your extensions work with Firefox 3 (after a restart, that is).
There’s also a portable version (yay) that you can use without any impact on your computer.