Best "Swiss army knife" for FireFox. Rated 5 out of 5 stars
This plugin as so usefull i wonder why it aint in the default FF install.
No matter if you are a hardcore developer, a beginner or just an occasional curious user:
This plugin will show you information about webpages that makes you addicted to it!
If you dont care about inner workings of a webpage then stay away of this.
But you are ofcourse otherwise you would not be reading this :P