The mozconfigs for recent mozilla-central clones are located in the browser/config/mozconfigs directory. Edit those as you please.
Note:
* TryServer purpose is to tell what will happen on Tinderbox, not to check every possible build option/configuration.
** Any non-standard feature is implicitly unsupported. You may try them, but don't complain if they break.
=== Old crufty way ===