What if the Mozilla builders was were better at managing all the different working directories (from Mercurial checkouts) that we need at any give time? If you look at [https://bugzilla.mozilla.org/show_bug.cgi?id=589885#c11 this conversation from IRC] you can see the benefits of this and [https://bug506404.bugzilla.mozilla.org/attachment.cgi?id=476270 a patch] that has the initial work.