Out of work that I'd been doing elsewhere in Mozilla, a while ago, I created an example repository for WebExtensions.
The intent was to bring together some ideas from elsewhere, and have set-up test harnesses that other developers could then copy and easily hook into their codebase. Slightly bigger description here.
You don't necessarily need the whole repository layout - just take the bits you want. If you find it useful or have suggestions for improvements, I'm open to pull requests and issues.