Among the new features is rotation simulation, basic geolocation API simulation, manifest validation, and a number of stability fixes for installation and updates to apps.
Also in the build is a new version of the Firefox rendering engine and an update version of Gaia, the UI for Firefox OS.
There are numerous other new elements (including a much faster start time and much smaller download and installation size) but another noteworthy addition is the ability to push apps to a phone, providing you have a phone handy that is compatible with Firefox OS.
In order to check out Firefox OS, you'll need to have Firefox installed on your system as the simulator runs as an add-on. To access it once installed you need to go to 'Web Developer' in the options menu and then select 'Firefox OS Simulator'.