Rated 5 out of 5 stars

TimeTracker 1.2.5 with Kenoubi's changes (thanks a lot!) working with FF4.0:

http://hotfile.com/dl/112924019/b83f225/timetracker-1.2.5-fx4.0.xpi.html
Don't forget show Add-on bar (Ctr+ / )

I also recommend TotalToolbar extension (http://totaltoolbar.mozdev.org/) for those who would like have counter on the top of the browser (example: http://img840.imageshack.us/i/timetracker.png/)