Firefox 1.5
Firefox 2.x
Firefox 3.x
Windows
Mac OS X
Linux
"TimeTracker is a Firefox add-on which tracks your browsing time across multiple sessions, letting you know exactly how much time you actually spend using the browser. Configuration settings allow you to disable the counter while you are idle or while you are browsing specific sites. Many of us tend to stray from what we initially set out to accomplish while surfing the Web. This timer can help out a great deal when it comes to maintaining your focus. It can also be useful for entertainment purposes."
"Do you spend too much time on Firefox? Do you open tabs faster than you can close them? Cannot get things done? Keep track of how much you browse with TimeTracker."

