Selenium is a free Web Application Testing System which has gained a lot of popularity since it became available. Its integration into 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.
Selenium also offers writing the test scripts as JUnit tests. With a special dynaTrace JUnit Runner and an extension to the Selenium base functionality it is now possible to tag web requests that are executed by a Selenium Java Test.

Find further information in the dynaTrace community