I reviewed the code of version 1.20. It looks solid. The required link injection uses some very specific DOM elements and may break when Google changes their search result page too often.
P.S. You can download the code with right-click > "Save as ..." from the "Add to Firefox" or the "Remove" button. The file is a zip archive (rename, extract, then open the folder in your IDE for review). Not sure why Mozilla does not make the main scripts of extensions more reviewable here.
