It'd make Firefox Sync between my Mac, iPhone, and iPad useful. Sure, it technically exists already, but Firefox on iOS is an annoying proposition since links can't open it.
My bookmarks stay synced between all three browsers on my computer and phone.
I'm relying on Xbrowsersync for now but it's still not the optimal "everything everywhere" solution, as it does not support syncing to Safari (and from reading their website, its developer appears to be ideologically opposed to implementing it).