Selenium [http://seleniumhq.org/] is a web testing automation tool that allows developers to automate the redundant bits of front end testing. The W3C wants to make this tool a standard so that more tools and test suites can integrate it with the confidence that it isn't going to go anywhere. This is not something that is for the average user or to power your website - this is just something that gets thrown into the build/testing process to ensure your app/site/plugin works as expected.
Now, rather than Selenium have browser specific code in the core executable, it relies on drivers for every browser you want to use. Mozilla has announced Marionette, which is a beefed up selenium driver that makes it easier to test the browser itself, rather than the browsers contents (think of automatically changing settings - turning off js, toggling plugins, etc).
This is in no way analogous to Active X because it is not meant for end users - just developers.
