Its been 12 years since Mozilla's "fireBug" dev tool has been around. Mozilla has decided to kill fireBug. This tool has been replaced with developer tools inside Mozilla developer edition Quantum (version 57). fireBug tool is still an active tool used by millions of developers.
Next month fireBug will no longer be available for use.
Mozilla Quantum Developer Edition
I highly recommend using Mozilla's new developer edition called Quantum. It has been released about a few weeks and has a new CSS grid feature. There are enhanced developer tools. I think you will be impressed with their new release. They redefined their user interface using their "Photon project". There are lots of extensions added and built in new integration's into their new developer platform!