Create a branding extension for browserconfig.properties and region.properties
In #3944 (closed), we went with a super-hacky way of updating omni.ja and the language packs to set our homepage and omnibox values.
It turns out you can do this same stuff from a branding extension. Tails has one here: http://git.immerda.ch/?p=amnesia.git;a=tree;f=config/chroot_local-includes/etc/iceweasel/profile/extensions/branding@amnesia.boum.org;h=0cf86e91aff4be8c98841e250d3eb9c943560257;hb=HEAD
We should switch to this as some point and remove all the crazy sed and re-zipping cruft from our TBB makefiles.