Extended Statusbar is another useful firefox extension which adds functionality to firefox. It displays additional information about the website that is currently loaded. Those information include the overall load time of the website, the amount of bytes that have been downloaded, the speed that you downloaded those bytes, the number of loaded images and finally the percentage that the site is loaded.
This is useful information if you want to find out why a website is loading slowly for instance. Webmasters are also able to check on their websites and see how fast it really loads.
source: lifehacker

Firefox Extended Statusbar
Extended page not found extension for firefox
Add And Edit Cookies Firefox Extension
Firefox View Dependencies
C’est clair, cette extension, sans être extraordinaire, est vraiment très utile !! Adoptée depuis longtemps