Do you have downloaded MeasureIt ? Installed it and restarted firefox but its not showing up ? Are you used to have this extension on the old firefox but since updating to firefox 4.0.1 but can't get this add-on which you loved. Here I summarized some tips to bring back your lovely add on.

Once you have installed, and the web browser asked, you must restart it. Even after doing that, your the MeasureIt ruler did not show-up on your Add-on Bar, if this happened to you have some problem with me, I found some method to solve it  from the verbatim abovementioned site.