Excellent idea, works well Noté 4 sur 5 étoiles
Only found out about this yesterday after submitting my own script to userstyles, doing a very similar thing, but this' still a smart solution to have more room for the urlbar.
My script cant autoresize, but has expand on hover and is still partially compatible with this, if you choose clear search query after submitting, which isnt the best trade off cause checking last search query is why youd want expand on hover most of the time
Anyway added instructions to use that script with this addon installed if anyones interested to try it. In practice the main difference other than the trade off is when its shrunk, you see the active search engine icon instead of this addons Go button. Can check it out here http://userstyles.org/styles/96225/search-and-url-bar-tweaks