In addition to the big two extensions that almost all developers who work in Firefox have installed (i.e. Firebug and Web Developer), another very useful Firefox extension which I like having tacked on is MeasureIt, a screen ruler measuring system developed by Kevin Freitas.
While technically the awesome Web Developer extension also comes packing a measuring ruler (Miscellaneous -> Show Ruler), I prefer MeasureIt’s simple, focused implementation, whereby you simply click on the ruler icon in the status bar, measure your target area by means of a standard mouse drag selection, view the dimensions of the selected area (in pixels) right next to the highlighted block, and then click on the ruler icon once more to close the whole thing down.
Simple, targeted and very easy to use.
You can manipulate your selection in terms of size and placement, making it an ideal way to get a feel for how much space you have available, or what is the best sized image suited for an area. In short, MeasureIt is really good at… well measuring stuff.
Well worth checking out if you are a web developer who enjoys working in Firefox.