If you hear Balrog, you are probably thinking Lord of the Rings, or maybe even of the Street Fighter character of the same name (also known as M.Bison), but certainly not of Firefox's update engine.
Mozilla used the same update engine for years and while there have been talks every now and then an update to the engine, it has only recently been made a priority by some members of the engineering staff.
The update engine is responsible for the distribution of updates to Firefox users worldwide. Considering that there are hundreds of millions of Firefox users out there that need to be updated in a short period of time, it is fair to say that it has done considerable well in the past.
Problems arose when Mozilla switched to the rapid release process as it not only reduced the time period between releases, but also added new channels which also required updates.
New requirements came along that the update server was not designed to prepare to handle. From throttling OS-specific update rates, the handling of custom updates for custom-build users of Firefox, or dropping support for operating systems or variants of it.
Mozilla update server
That's why work began in earnest on the next version of the update engine. Development was switched on and off for years but more important projects put the development on ice for a while, before it was resumed.
Mozilla will launch the new update engine Balrog for Firefox Nightly users today. If things turn out alright, end users won't notice a thing. Updates will be delivered as usual and that is it.
The engineers responsible for the new code have tested the new update engine extensively, but there is always the chance that something goes wrong when code gets deployed in a productive environment.
The worst case scenario is that the update breaks Firefox Nightly in a way that cannot be repaired remotely, say by distributing a second update that fixes the first.
Your best bet if that happens is to download the latest Nightly version from the official Firefox website and install it anew on the system.
The update itself is not really that interesting to users of the browser. In best case, no one will notice the switch as updates will be handled just like before. For Mozilla, it is a big step in the right direction as it gives the organization better control and management options in regards to updates for the Firefox web browser.
Additional information about the update are available on John O'Duinn's blog.