I found my solution here

Here are the steps: 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.

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). 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!.

The above is verbatim from the abovementioned site.

Hope this helps.

Thank you so much. I have really missed this little tool!

Thank you very much.Its really helped me .

Thanks it helped me too....

thanks alot. it was so great ,