Speaking of tracking, it is now easier to enable tracking protection. Go to about:preferences#privacy
Yes, it is using Disconnect’s lists to block trackers. Many of times it blocks tracker adverts.
Makes pages load faster than before.
Read: Tracking Protection Study
Great for Firefox users, no need to use any other add-on.
The sandbox level is 4 in Firefox 58 (Beta) and 5 in Firefox 59 (Nightly). The latter is not yet documented at https://wiki.mozilla.org/Security/Sandbox
To see the level, go to about:support. The level can be changed. Go to about:config and search for security.sandbox.content.level
Attached is an image of Firefox 59 (same sandbox as 57 on Linux) on Ubuntu 17.10.
Firefox 59 has a nice new feature: it supports AOMedia Video 1 (AV1). You can watch Tears of Steel in AV1 here: AV1 Demo by Mozilla and Bitmovin
Don’t expect great performance when watching 1920×800. AV1 is quite complex, and the software used to decode it is not yet very well optimised.