Do you ever wonder how programs make it through the testing process? Mozilla’s decision to speed up development of the Firefox browser gives users an unique opportunity to get a close look at this process. But despite the much-vaunted emergence of developmental builds like Firefox Beta and Firefox Aurora for wider public consumption, there’s still a lot of testing to be done even before a version of Firefox reaches alpha, or Aurora, status.
This testing is done using nightly builds – developers make changes to the code during the “day”, and then at “night” a fresh build is uploaded to the Mozilla servers for developers and hardcore enthusiasts to test. It’s important to know at this point that many of the code changes have not been tested – they’ve been incorporated into the main build, but that’s it. It's now up to the hardy souls who run Firefox Nightly to try out the changes and report back on any stability issues they encounter.
The Nightly build will install alongside other Firefox builds, specifically the Aurora build and the Beta/Stable build. However, it will share access to existing Firefox settings, meaning there is a risk you could lose these if something catastrophic was to happen.
With this in mind, we’d strongly recommend you only install Firefox Nightly on a test machine, or in a virtual environment such as that provided by VirtualBox. Once done, you can safely road test it, secure in the knowledge you’re not going to screw up your main computer by doing so.
Nightly introduces Firefox 39.0a1. You can see what's planned for this version by visiting the Fixtures/Release Tracking page, although note that most of these features are not implemented in Nightly. One interesting point to note is that Nightly also offers a 64-bit version of Nightly for Windows as well as Linux.