Here's how to use the built-in CSS query selector in Firefox:
Go to Tools > Web Developer > Web Console
Also, you could press ctrl shift i in Windows/Linux, or cmd opt i in Mac.
Type in your CSS selector (using traditional $$() syntax) at the very bottom left corner.
The object node list will appear on the right-hand panel of the console.
$$('div') [object NodeList] $$('div').length 42
This is handy for Selenium Webdriver instances of Firefox, were having an extension isn't feasible.