If you want to play about on Firefox OS on an actual device you are going to have to wait a while, but if you want to test out some of the applications that are already being built for it and you have an Android phone you are in luck.
Basically you need to install the Firefox nightly (named Aurora) and it has access to the Firefox OS marketplace.
- Click this link to install Firefox Aurora
- Launch the Aurora app.
- In the Menu go to Tools >> Apps
Apps you install go onto your homescreen, which gives you a nice experience when you are testing/playing.