Selenium is not an independent tool, you need to use it with the support of some programming language like Java, Python, etc.
There is a alternative that I have used it is called as Watir. You can use it as its a open source testing tool. The UI is good and the libraries are based on Ruby.
With the help of Watir you can do cross browser testing including Firefox and Opera. It also supports data-driven testing and integrates with BBD tools like RSpec, Cucumber, and Test/Unit.
Hope this helps.