It has the following changes:
1) Added support for CSS animations. Making this type PSD to CSS conversion a lot easier.
2) Added support for switching Firefox development channels
3) The Do-Not-Track header preference has been moved to increase discoverability.
5) Improved standards support for HTML5, XHR, MathML, SMIL, and canvas.
6) Improved spell checking for some locals.
7) Improved desktop environment integration for Linux users.
There is little to differentiate Firefox 5 from Firefox 4 in this beta release, but it does feature the new Firefox Channel Switcher that allows users to move between developmental and stable versions from a convenient dialog box.
You might think what had its developers being up to? They are haunting down bugs and need all the help from PSD to HTML coders they can get. This list covers some of the known problems with Firefox Beta:
1) All Systems- If you try to start Firefox using a locked profile, it will crash.
2) Microsoft Windows- Some users of Adobe Reader X have experienced instability when viewing PDF documents in the browser.
3) Mac OS X- This version of the beta will not work on Macintosh hardware with Power PC CPUs.
4) Linux and Unix- The video control buttons may not work when viewing QuickTime videos with libtotem.
Also remember that installing Firefox Beta will overwrite your existing installation of Firefox. You won’t lose any of your bookmarks or browsing history, but some of your extensions and other add-ons might not work until updates for them are made available.
The final release might be ready for late June, at that time we will also see the dead of Firefox 3.5 because Mozilla will forcibly upgrade the 12 million remaining users. If you are one of them we advise you to say your good bye and joint the new improvement of the Firefox’s pack.