So since the earlier release, I've been working on switching the code over to use the Firefox add-on SDK. The SDK will offer better and make it easier to implement functionality and the add-on will still be restart-less so that condition is still met.
So far the I haven't gotten far enough with the SDK to put an icon under developer tools, I have been able to get it to work with the add-on bar (shown below).
When the icon is clicked the add-on currently gets the current state of the DOM in tab of a specific window. The add-on shows the DOM in a panel in the centre of the browser (shown below).
I plan to make the panel read-only and eventually be able to publish to Webmaker through the add-on. I might integrate the Thimble editor itself into the add-on but that I'll decide late.