Love it, it's small and does exactly what it's supposed to.The one suggestion I have is adding a reset option to the drop-down when it is clicked, as that would be much more convenient than navigating to about:addons. That, or adding an option to choose when it resets - a certain time each day, or when the browser is closed. I personally would like to monitor each session individually, but I understand the benefits of monitoring the entire day.
*EDIT* I got to thinking about this, and the easiest way to implement such a feature would be to add a session timer on top of the daily/weekly timers that is more transient. It could be cleared when Firefox is quit, and there's really no need to archive data that specific. And then from there a timer selection feature could be added (which would also allow users to have a timer for the week easily visible without going to the summary). I don't want to push you to implement anything like this if you feel it's unnecessary, but I figured I would add more clarity to a possible implementation of my ideas.