Could you not use permissions.request() [0] to ask for access to a given site on-the-fly, when the user actually visits it? That seems to be how, for example, Reddit Enhancement Suite goes about getting perms for all the various sites it supports embedding content from (imgur, twitter etc) - you get a prompt when you click to open an embed for a site you haven't granted the permission for already.
[0] https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web...