Essential for the extension developer Noté 5 sur 5 étoiles
I've been using this extension for years now and am glad to finally see Ted finally moved it to AMO.
This extension is an extension developer's best friend. If you're an extension developer, this extension is a MUST-HAVE.
The JS shell (by Jesse Ruderman; included as one of several tools in this extension) with chrome privileges is essential to smooth extension development.
The real-time XUL editor makes screen prototyping dead easy. It's also great for sharing/debugging XUL with colleagues over email or pastebins.
Ted really needs to be commended for bundling these tools and maintaining this extension. There would be far fewer Firefox extensions -- and buggy ones -- if this extension didn't exist.