I just checked in a patch to build-system which changes our mozconfig detection logic. Specifically:

• If you want to specify an explicit mozconfig, the only way to do that now is to set the MOZCONFIG environment variable. We used to support the obscure MOZ_MYCONFIG variable too, but we now error out if it is set. Note that if MOZCONFIG is a relative path, it will be treated as relative to the current directory. (update: apologies, this is still broken).
  
• More significantly, we used to support a number of implicit mozconfig locations other than the usual <srcdir>/.mozconfig, such as $HOME/.mozconfig and a couple of variants. This went against build system’s policy to be as explicit as possible, so these locations are no longer supported, and if we used to pick up a mozconfig there we will now error out. The only implicit location still supported is the usual <srcdir>/.mozconfig.

This patch should make it into mozilla-central soon, well in time for Gecko 9. This patch will also affect comm-central in the same way