The Debian build of Firefox does load extensions from /usr/share/mozilla/extensions, so that it will load the extensions in the Debian webext-* packages. You can even add a symlink there pointing at a dir in your /home so you can load extensions you are developing.
That's because it's an ESR build. Normal build does that, too. The extensions still have to be signed. It's not a Debian thing.
One ESR build, you can disable signature checks though in about:config. Not sure how this fits into the standard Mozilla orthodoxy. Remember that core tenet of the orthodoxy is that users can't be trusted to protect themselves...
So maybe Mozilla cares less about safety of users that want to use their ESR (extended support) build. There are way fewer of these users than that of main Firefox build, so their safety is maybe not that important on the grand scale of 2.5% marketshare that Firefox still holds.
