It's been less than two weeks and we've gotten 30 downloads without really doing any promotion of the add-on (other than posting on IH!)
I've made statistics public so you can see them over here: https://addons.mozilla.org/en-GB/firefox/addon/growth-gecko-contacts/statistics/?last=30
Ok, I know it's not a huge amount, but it's enough to motivate me to work on a Chrome extension. Firefox only has around 5% market share, so hopefully a Chrome extension will be bigger.