Purpose of this page
The Mozilla build process and code base have evolved considerably over the past few years, this has resulted in many caveats and tips being added to the MDN documentation for building older versions of the code base or to deal with older build tools.
These tips while helpful are jarring and superfluous to the majority of new Mozilla builders who are mainly interested in building the latest version of Firefox.
Therefore instead of deleting all these nuggets of information it's best to collect them all here and remove them from the majority happy path documentation.
Each piece of information should mention the page and the section it originally came from.