TimeTracker is a Firefox extension that can help monitor the amount of time you spend online.
TimeTracker displays a small clock on the browser's bottom right corner with which calculates the amount of time you spend browsing the web. This information can be useful to check how much time you really spend on the web or to cut down your procrastination time. What's more, if you hover your mouse over the clock you'll see extended data such as your total browsing time since the extension was installed or since the last time it was reset.
TimeTracker can be configured to ignore certain websites, so for example it doesn't count your browsing the company intranet as a waste of time. Unfortunately time is tracked on a general basis and not according to each specific site. There's also no possibility of creating and exporting statistics, but according to the author's webpage, specific time tracking, a get back to work alarm and advanced statistics for export will be available in future releases.
TimeTracker can help you control the amount of time you spend browsing the web and increase your productivity.