As you may already know, Firefox is being developed on three separate channels. First, the features are implemented in the developer branch, they reach the beta channel when enough tests have been performed and finally, some of the new features from the betas get included in the stable version of Firefox.

Recently, Firefox 41.0 Beta 1 has been released, bringing improvements.
New features:
- SVG images can be used as favicons
- Support for TSF (Text Services Framework).
- New look for session restore and welcome back page
- Ability to set a profile picture for your Firefox Account.
- Firefox Hello now includes instant messaging
Changes:
- Type 2 Add-ons (Extensions) that have not been signed by Mozilla will be disabled by default.
- Read more about Add-on signing in Firefox
- Memory overhead of AdBlock Plus has been greatly reduced
- WebRTC now requires perfect forward secrecy
- Improved image decoding with performance up to twice as fast on some devices especially while scrolling
- Smoother and more reliable CSS animations via asynchronous animations
HTML5:
- CSS Font Loading API enabled by default
- Copy/Cut Web content from JavaScript with document.execCommand(“cut”/”copy”)
- MessageChannel and MessagePort API enabled by default
- Added support for the transform-origin property on SVG element
- Implemented Cache API for querying named caches that are accessible window, Worker, and ServiceWorker
Developer-specific enhancements:
- Screenshot a node or element from markup view with the Screenshot Node context menu item
- Copy element CSS rule declarations with the Copy Rule Declaration context menu item in the Inspector
- Network requests can be exported in HAR format
- Removed support for binary XPCOM components in extensions, use addon SDK
- “system/child_process” pipe mechanism for native binaries instead
- Pseudo-Class panel in the Inspector
- View HTML source in a tab
- Quickly add new CSS rule with New Rule button in the Inspector
Fixes:
- Picture element does not react to resize/viewport changes
For more information, see the official changelog for yourself.
Installation instructions:
Up to date packages are available via the Firefox Next PPA, so installing the beta version of the internet browser on Ubuntu is easy. Just add the PPA to your system, update the local repository index and install the firefox package:
$ sudo add-apt-repository ppa:mozillateam/firefox-next
$ sudo apt-get update
$ sudo apt-get install firefox
Optional, to remove firefox, do:
$ sudo apt-get remove firefox
The installation instructions should work on Ubuntu 15.04 Vivid Vervet, Ubuntu 14.04 Trusty Tahr, Ubuntu 12.04 Precise Pangolin, Linux Mint 17.2 Rafaela, Linux Mint 17.1 Rebecca, Linux Mint 17 Qiana, Linux Mint 13 Maya, Pinguy OS 14.04, Elementary OS 0.3 Freya, Elementary OS 0.2 Luna, Deepin 2014, Peppermint 6, Peppermint 5, LXLE 14.04 and Linux Lite 2.