You could do something like:
((JavascriptExecutor) driver).executeScript("return s.evarx;");
simply replace "return s.evarx;" with whatever javascript you execute in the Firebug console that you want to do with WebDriver. Most of the code you want can be executed this way, but not all.
Be sure to add a "return" keyword to the javascript code to execute if you are expecting to get the result/data. Otherwise, no need to if you are just executing code to do something.
An example that isn't helpful when executed via Selenium: "console.log('something to log');" unless you actually extract and deal with Firefox logs via Selenium, a separate topic.