AATW: “Could someone who knows his/her way around XUL please develop a Tumblr button for Firefox?”
Does the “Share on Tumblr” bookmarklet (Dashboard/Goodies) satisfy your needs?
Firefox is a reasonably good browser, and since it sucks less overall than the others, I use and recommend it. But as a previous Firefox extension author, I wouldn’t wish that duty on anyone.
I advise that you avoid Firefox extension development unless you need to accomplish something that absolutely isn’t possible with a different technology, such as a bookmarket.