Firefox Beta is basically a separate version of this popular web browser where you can try out the latest features and changes before they're roll out for the wide public. Being a test release, you should expect to run into issues and bugs from time to time and don't hesitate informing the developers about them. That's the main purpose behind this application.
Don't expect huge changes regarding the design of the interface. You can still open multiple pages at the same time and switch between them thanks to its use of the tab system. You can install various extensions to further expand the functionality of this well-known browser. Each time you find something really cool and informing, you can save it as a bookmark and even share this content with other people through Bluetooth, Facebook, Twitter, WhatsApp, and so on.
Thanks to this release, now you're able to access a variety of performance improvements, security updates, customization options, new or changed features, before they are pushed in the main version of Firefox.