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, where having an extension isn't feasible.

Yes you can go for FireBug, a versatile Firefox web development add-on.

_{(source: getfirebug.com)}

To test a CSS selector, go to the "Console" tab and enter a command in the bottom form (more info on how to find the command line).

Inside the command line use the $$("your CSS selector") syntax to test CSS selectors, explained in more detail here. For example use this command to select everything:

$$("body") 

Try firebug. http://getfirebug.com/

Not sure if this helps. Try Firebug. Allows you to select an item, and see what it's css path is, as well as the css currently being applied.

Can do some experimentation in the html/css right in the browser.

FireFinder is good, but I started with and prefer FirePath for Firebug. It works similarly, but can give you an expanded view of the HTML around the matching elements w/o need to click inspect, FriendlyFire, etc.

The field where you test the locator also has syntax checker where field turns red if syntax is bad. Just click eval to test the locator and matches show below with additional HTML around the matching elements.

But for testing CSS locator, you'd want the drop down option of "Sizzle" rather than CSS in FirePath. The CSS option only works for simple CSS selectors, complex ones only work under Sizzle (l mode, such as:

div.namedService.photoService > div.photoBrowserContainer:nth-child(3) > div.albumName:contains('someName')

Selenium IDE 1.0.11 released has inbuilt CSS locator builder

The DOM standard function document.querySelectorAll is what you want, modern browser all support it. See the document

https://developer.mozilla.org/en-US/docs/DOM/Document.querySelectorAll

You can call it in built-in web console. In console there is a shortcut $$, call it like $$('div a').

I like firebug because it can click to scroll to view the element. It also can test in 'CSS' panel.

The 'Find' button in Selenium IDE is very useful for this. It uses the same method to locate elements as your tests will, so can be used to locate elements using any of the supported strategies.

jQuery

With jQuery you could easily add a large red border to an element using the selector.

$(document).ready(function(){ $('#your-css-selector').css('border', '5px solid red'); }); 

Firebug (https://addons.mozilla.org/en-US/firefox/addon/1843) or Web Developer Toolbar (https://addons.mozilla.org/en-US/firefox/addon/60). Both show path.

Firefinder is great for testing selectors. However, if also you want to obtain the CSS selector for an element try SelectorGadget.

I've found FirePath to be really great, it lets you look up not only CSS but xPath as well. Wish there was something similar for Chrome and IE, but alas!