Firefox DevEdition has a feature where it uses a different profile from normal-Firefox and it allows itself to be run in parallel. If we're to make DevEdition a repack (bug 1459240), we'll need this to be the sort of thing we can enable/configure in a repack. Right now it looks like this is largely handled via various "#ifdef MOZ_DEV_EDITION" chunks within SelectProfile() in... toolkit/xre/nsAppRunner.cpp toolkit/profile/nsToolkitProfileService.cpp ...and probably other places. So those parts of this behavior are locked in at compile time, via the #ifdefs. To make this repack-friendly, we'd need to swap out those #ifdefs for something more dynamic that can be toggled via a runtime config file of some sort.

Some relevant source links for some of these #ifdefs, for reference: https://dxr.mozilla.org/mozilla-central/rev/5866d6685849311f057e7e229b9ace63a2641c29/toolkit/xre/nsAppRunner.cpp#2518-2538 https://dxr.mozilla.org/mozilla-central/rev/5866d6685849311f057e7e229b9ace63a2641c29/toolkit/profile/nsToolkitProfileService.cpp#523-550

Tromey posted a patch on an earlier "DevEdition as repack, maybe?" bug that seems to largely address this -- it's the second commit (of 2) on Bug 1354262's MozReview page: https://reviewboard.mozilla.org/r/129568/diff/ (scroll down to the nsToolkitProfileService.cpp changes) Looks like that patch had r+ modulo a commit-message nit (and didn't land because we didn't end up going for a repack at the time).