Firefox's mozconfigs have lived in RelEng's buildbot-configs repo since the dawn of time...well, at least as long as I've been here, so that's a few years at least! With our new rapid release schedule and the explosion of project branches, it's become much more difficult to keep the mozconfigs in sync with code changes across merges between branches. It's always been difficult to coordinate landing code changes with mozconfig changes; and since mozconfig changes happened out of band from regular source code changes, they were invisible on tbpl. We've recently changed the build automation so that mozconfigs will be first checked for in e.g. $topsrcdir/browser/config/mozconfigs/$platform/{nightly,debug}. If this file exists it will be copied into $topsrcdir/.mozconfig, otherwise the original mozconfig will be fetched from hg. There are two major benefits to this change:

• mozconfig changes are now a distinct point in the development history of the product itself. This means that changes to compiler versions, optimization flags, etc. are tracked along with regular code changes. Changes to the mozconfigs will trigger builds and tests like any other code changes.
• Changes to the mozconfigs will automatically be carried over between branches during a merge. If you change the compiler used in mozilla-central, then the next time we merge mozilla-central into mozilla-aurora the change to compiler will be merged as well.

Of course this means that if your branch has special settings in your mozconfig, then you need to be careful when merging back to mozilla-central or mozilla-inbound. We'll also have to ensure certain changes like --enable-js-diagnostics are turned off when we move from mozilla-central to mozilla-aurora. In particular, on your first merge from mozilla-central, any custom mozconfig settings you have will be lost. I'm planning on landing the mozconfigs into mozilla-central early next week. As always, if you have any question, comments, or concerns, please let me or anybody else in RelEng know! For all the gory details, see bug 558180. UPDATE: Branches currently using the generic configs (eg try, places, twigs) will lose support for mozconfig-extra[-<platform>].