I've gone over the Mozilla Developer Network "Getting Started (jpm)" guide to getting started with Firefox add-on development.
This guide suggests that to see the effects of changes that I make to my add-on's code I need to:
Do I really have to rebuild my package and run it a new instance of Firefox to see my changes?
Is there an alternative way to refresh the add-on code from within the browser?
You can combine jpm with the Extension Auto Installer add-on like this:
jpm post --post-url http://localhost:8888/
Or using watchpost to automate the process:
jpm watchpost --post-url http://localhost:8888/