Rated 4 out of 5 stars

"Only" four stars because it is so easy to quit the chat. You should use a small icon (like in gmail) to quit or switch to the gmail site instead of all the bar... it irritates me ><, so I never use the chat.
EDIT : could you make the popup draggable instead to have a link on the title bar

This review is for a previous version of the add-on (0.1.9.1-signed).