I had the great pleasure of attending the Maemo/Mozilla Danish Weekend last weekend, where I gave a talk on user-interface issues and tips to think about when creating add-ons for Fennec. The high point for me, other than getting suggestions from some of the first people to try building add-ons for Fennec, was getting to see the results of the Fennec Add-ons Developer Challenge, including the winner: Fabrice Desre's GraffiTwit.
Brian King wrote a great overview of the event, with links to the winning Fennec add-ons, so I'll cheerfully refer you over there rather than recounting everything again.
I will link to my slides, though. Here they are: