Selenium is a free Web Application Testing System. It gained lots of popularity since it is available. It’s integration to FireFox – which allows instant record/replay as well as the scripting option - makes it a great tool for functional web testing.
BrowserMob on the other side leverages Selenium by providing an automated load-testing environment for Selenium scripts all configurable online via a Web Dashboard.
Read the whole article at http://blog.dynatrace.com/2009/08/07/seleniumbrowsermob-integration-with-dynatrace/