Not sure why you're being downvoted, seems like a legitimate question to me! I was a little leery of checking it out at first, too, but curiosity got the better of me...

Maybe someone who has developed a FF extension can lay my worries to rest -- could this have, say, a built in key logger which sends that data to the author?

I downloaded the source code from github and glanced through it, enough to comfort me somewhat, but I wasn't super thorough.

All that said, EricButler seems to be an HN member in good standing, and someone like that wouldn't do anything malicious, right? :-)