As a user of sid who also maintains various systems running stable, I
rely on packages like xul-ext-adblock-plus to make it easier to install
specific addons systemwide. I find it much easier to install those via
the Debian packaging system rather than a user-level mechanism that
involves running Firefox as one or more target users (or more likely
doing the equivalent of creating a xul-ext-* package for local use). I
realize that you can't maintain the full library of Firefox addons as
packages, but I'm hoping that some of the most common and popular ones
stick around and stay up to date, notably Adblock Plus, HTTPS
Everywhere, and It's All Text.