By default you can't measure it in pixels no matter what web browser you're using.
|Easy to use... and excellent as well.|
But if you're using Firefox version 1.0 to the latest 4.01 version, then there is an excellent addon called MeasureIt. It did not support the latest Firefox version 4 until recently but now you can use it in the version 4 nonetheless.
Installation is pretty simple. Just go to this measureIt official mozilla page and click on "Add to Firefox" button and follow instructions on your screen.
Once installed, when the web browser asked, you must restart it. Even after doing that, in my Firefox4 the MeasureIt ruler did not show-up, if this happened to you (it happened to me) then use the below method.
1. From the main menu in Firefox4, go to: "View" -> "Toolbars" -> "Add-on Bar" (just make sure it's enabled - you should see a "check mark", if not, simply click on it).
|Before enabling the Add-on bar (it's disabled by default if I'm not mistaken).|
|After enabling add-on bar (in Red is the add-on bar and in Green is our MeasureIt icon).|
Now even after enabling it still if you can't see a that Yellow MeasureIt icon, then again go to : "View" -> "Toolbars" -> "Customize" and from the toolbar window you get, scroll down till you find the "MeasureIt" icon and simply drag it and drop on the above mentioned "add-on bar" (in Red of above screenshot) section in your Firefox4 browser. That should do it!.
If you use Safari Web browser or the Google Chrome then no worries... it's also available for those browsers as well. Get it from this official author page.