It's silly that to turn some feature on across builds, one needs to edit a bajillion different mozconfigs. There should be a file(s) somewhere in $topsrcdir/build/ that contain common settings like these (mozconfig.official, mozconfig.nightly, etc.) that can be assembled so that relevant changes only have to be made once.
If certain configs *don't* want features that are turned on by these common files, then they can explicitly turn the feature off.
I'm sure RelEng will be interested in this.