The website addons.mozilla.org recently changed the rule for sandboxed addons: you no longer need to register and login to install one. That was a big issue because the review process is a bit heavy, and a lot of add-ons were stuck in that Limbo of Firefox.
The difference for my recent sandboxed Video Games Spy (a sidebar to get aggregated info about games) is huge. It went from 0-ish to more than 50 downloads a day! It’s still not a lot, especially compared the thousand a day Moji is still getting, but it shows there is some interest for the add-on.