Joey Minta has put together some API ideas for STEEL, a FUEL-like library for Thunderbird. I share David Ascher’s feelings that such a library would be a fantastic way to simplify Thunderbird extension development and get more people building Thunderbird extensions.
Personally, I’d like to see if we could move some FUEL interfaces into the toolkit level instead of duplicating the code for each application. Also, we could look at making a simple, general base interface for some things (like Window) and then override the base interface as needed (think BrowserWindow and MailWindow).
Add your thoughts to the newsgroup post and/or the wiki page.
I put together a rough cut at how one might move the toolkit parts of FUEL into an implementation that could be shared, and then one could expand the Application object. It’s quite rough and not actually workable at this stage, but you can get the idea, based around a __proto__ object: http://pastebin.mozilla.org/250378