This add-on is specifically designed to provide instant access to Google Contacts directly from the bookmark bar in the same manner as two other add-ons from this developer, Integrated Google Calendar and Gmail Panel . No need to dedicate a tab - each app opens quickly with one click & disappears when you click on anything outside of its page, The three add-ons have several great common features. Via the add-on's options, you can specifiy the size of the page that pops up, set control keys, etc. If activated by an icon in the upper right of a page, a pin feature prevents the page from closing as it normally would.
I use all three throughout the day - every day and as such, I would have to say that these are the most useful additions to Firefox.