They're also working on tracking protection similar to IE.
There's been a bunch of work related to how Firefox draws and stores images, a lot has landed already and it lays the foundation for "downscale on decode", where instead of storing the whole uncompressed image in memory and then downscaling for display, it'll downscale it while decoding and only store the screen sized representation of it. On a desktop that's not going to help much, but on mobile you go from 1GB of images to 30MB, etc.
Also, the work will allow for icons to scale themselves, i.e. displaying an ICO file at 32x32 will show the 32x32 variant, vs. the 16x16 variant being upscaled. Quite useful for stuff like folder browsing, the download list, preferences window, favicons, etc.