Hello,
There are lots of free downloadable extensions and add-ons available on the internet for firefox and day by day demands are increasing. Somethings are better if they don’t get uninstalled from your firefox. In fact, they can make your firefox more efficient a. We give you the 15 Firefox addons that a CSS developer should think twice before uninstalling because they will make your life as a CSS Developer much easier. Read more at 15 Useful Firefox Add-Ons To Make CSS Developers Lives Easier

Let me know, if you know any other valuable add-ons.
Regards,
Mostafa Anoosheh