I used to use an add-on for firefox called measureit. Now its not working because of firefox keeps updating all the time and the developer hasn't come out with a version of measure it that is supported by newer versions of firefox. Does anyone know a good pixel ruler add-on for FF?
You could download the web developer toolbar. It has a ruler built in. Once you install it, it will appear in your menu bars in firefox. Click on miscellaneous, then click on display ruler.
You could also try searching "pixel ruler".