Because Firefox OS is currently under active development, and is in a prerelease state, the best way to ensure you have an up-to-date copy installed is to build and install it yourself. The articles listed on this page will guide you through building and installing Firefox OS on an emulator or compatible device, or the Gaia user interface in the Firefox browser.
Building and installing Firefox OS requires a significant amount of time, network bandwidth, and computational power. This page outlines the goals of the build process and the steps of that process in order to help users along the way.
A guide to running and using the Firefox OS desktop client; this simulates the Gaia environment in a desktop application. It's more accurate than running Gaia in Firefox but not as accurate as the emulators.