The Classic Firefox Add-on Rated 5 out of 5 stars

It's no longer developed, but it's already a classic, doesn't matter it is a legacy addon, it works perfectly on Firefox 8, stable and light, and I use it not only before capturing a cluttered webpage (using Screengrab, happened to be another classic), but also for creating element hiding rules of Adblock Plus. Of course, also works as an html inspector.