Exciting us about Firefox 51 Beta is that WebGL 2 is now enabled by default. This comes as Google also just enabled WebGL 2 by default in Chrome/Chromium. WebGL 2 is based off OpenGL ES 3.0 and is a big upgrade over WebGL 1.0. WebGL 2.0 allows for higher precision shaders, more textures, and other upped graphical capabilities to allow for more visually rich applications and games in the web browser. The WebGL 2.0 specification is currently considered an "editor's draft" by The Khronos Group and was last updated earlier this month.
The Firefox 51.0 Beta that was released yesterday also has improvements for being able to save passwords for forms lacking a submit button, improved video performance when not using GPU acceleration, the zoom indicator is now shown in the menu bar, and there is native support for FLAC audio.
The Linux Firefox build also now makes use of Skia for content rendering. The Skia Graphics Engine is what was open-sourced by Google years ago and is also used by Chrome.
More details via the Mozilla.org announcement.