Measure it Firefox add on - Ruler Extension for the WebDevelopers

Measure it Firefox add-on shows you the accurate measurement of any part of the page in pixels.

Use:

Comes in handy if you are a web developer or a blogger who makes changes in the design.

How to find it?

After installing this add on if you did not find any ruler icon in the add on bar or the tool bar, then go to toolbar layout in the menu button as shown below.

Getting Customize Toolbar window from the menu

You will find Customize Toolbar window as shown below.

MeasureIt Firefox add on in  customize toolbar window

Mozilla recently updated, customize toolbar window in Firefox 12, so you may find the window to look different if you are using Firefox 11 or below

How to use it?

You can find the measurement icon in the left side of the add on bar.  When you click it, the page you are viewing will become semi transparent as though there is a new layer above it.  Move the mouse to the place you want to measure and start measure it.

Measure any part of the page using Measure it Firefox add on

After you have finished measuring re-click the icon in the status bar or press 'Esc' and you will get the page back to normal.
If you want another ruler to measure and not the one you are viewing, just click the close 'x' button at the end of the ruler.  You can drag Ruler icon any where you want.

Some useful keyboard shortcuts:

CTRL + Arrow Keys to move ruler by 1 pixel.
CTRL + SHIFT + Arrow Keys to move ruler by 5 pixels.

Google also recommends this Firefox add-on to measure the size of the Adsense ads.

Also check out Colorzilla Firefox add on, Pixel zoomer firefox add on and other web development add ons.

You can use add on compatibility reporter to report any incompatibility issues when using this add on

Install this Firefox add on from the Mozilla site.  Please share your experience through comments.