This extension uses the google charts API to generate the QR code, meaning it's NOT self-contained and effectively "leaks" the URL to google whenever you use it.
This isn't bad by itself, except that the title being "JavaScript" can be easily interpreted (such as was my first thought) to be purely done in javascript without performing external web requests to 3rdparty services.
It is a nice lightweight extension regardless, but I can't rate it too high due to this.