The first beta build for Mozilla's browser, Firefox 4, was just released today with a variety of changes. First and foremost, check to make sure your crucial add-ons are currently compatible with the latest build. In this case, that would be beta1. If all is still well, then install and prepare for the changes (only two of them). The first difference is that tabs are now on top of the window, similar to the layout the beta release for Safari 4 used. If you weren't a fan of them, there is an option for you to return them to their familiar home.
The second is the introduction to the "Firefox button". Instead of a menu bar, you get a button. If you've ever used an app from Microsoft Office, then you know what I'm talking about. Just like the tab change, there's an option for the menu bar to return (which I will be doing).
For now, these changes will only be available for those that use it on Windows but will later be available for Mac and Linux users. You can get the beta here.