Honestly, the absolute best thing you could do for any dev, is lighten the footprint FF currently has on my ram, and continue to improve the api for add-ons/plugins. The more stable, extensible and fast FireFox is, the more useful I will find it for development. But if more kludge is added to it, and it continues to get slower/bigger, the less likely I am to continue to use it, regardless of what amazing plugins are…
We are also looking at existing extensions.