Rated 5 out of 5 stars
Do you have a github repository for this? I was hoping to team up if you are comfortable.
The additions I was hoping to add was:
* Rather then toolbar button, when user holds mouse button down it will zoom. If they continue to hold it will zoom one element deeper, until they stop holding.
* Holding while zoomed in will zoom all the way out
* Options page (padding on zoom, mouse hold time)
* Make it work in non-html pages as well
Here is my addon and github repo: