Earlier quoted context omitted.
Huh? To the best of my knowledge, to get a development extension working in FF: 1. Ctrl-shift-a to open add-ons page 2. Click the cog, and select "debug add-ons" 3. Click "load temporary add-on" 4. Select a file in your extension. 5. Done. This worked for me earlier this week.
Key word from the grandparent post: "permanently" Key word from the parent post: "temporary"
Ah missed that; yep, the add-on will remain installed until the browser is restarted. That is a little annoying!