In case you missed it, I gave a presentation on the state of Firefox's build system yesterday.
If you build Firefox from source regularly, you should definitely at least skim through the slide deck.
I'm not an HTML expert, so my apogolies for bad UI on the interactive slides. You may need to press enter to select items in dropdown menus. Also, the interactive slides are a bit resource intensive. If the slide deck is really slow, turn off those elements. I've also only tested the slides in Firefox 19 and 20. My apologies if they don't work everywhere.