Timetracker for Firefox
Timetracker for Firefox will tell you how much time you spend using the browser over the course of the day. It also provides a running timer on the total time you’ve been using the browser since you installed the tracker.
Beyond this basic information, this Firefox extension isn’t going to help you much. It doesn’t provide browsing information on a site-by-site basis or anything like that. This is just a basic browser stopwatch, plain and simple.