So I can understand how much time i spend on social sites like facebook or flickr, how much time is on mails. etc
3 Answers
1) MeeTimer
It records where you spend your time online. It does it in a rather useful way, by allowing you to group websites into activities
2) TimeTracker
Features:
Tracks browsing time across sessions
Pauses automatically when you stop browsing
Reset counter
Pause toggle per window
List 'work' sites that should not count
Tooltip showing total time today, since last reset and since installing.
Options Window
I think that pageaddict will do the trick
https://addons.mozilla.org/en-US/firefox/addon/3685
But it seems it is only working with FF3.0