No promises that that's actually the right flag. I had a rummage around searchfox and it looks like that just enables extensions that have been placed in specia

No promises that that's actually the right flag. I had a rummage around searchfox and it looks like that just enables extensions that have been placed in special directories (whether they must be signed or not is a different flag). There clearly is a setting somewhere though as the unbranded builds exist...