I imagined that the extension would allow me to edit the PDF at the URL that's in the browser.

Instead, the extension seems to be primarily a link to http://www.redcoolmedia.net/onlineeditor/preloginpdfoffice.html

Maybe a great service (I didn't test it) but version 1.3.5 of the extension seems to do little more than can be done with a simple, traditional bookmark.