There are some other ways also to install Firefox in Kali like linuxmint debian repository, PPA etc. But the method i am sharing is the best and works 100% If you want to install the Firefox in Kali run the following commands in your terminal remember it will first remove the Kali default browser Iceweasel. Iceweasel is Firefox only with different name and logo. But for some people it doesn't give a real Firefox experience. I also removed the Iceweasel and running the Firefox

apt-get remove iceweasel
echo -e "\ndeb all main" | tee -a /etc/apt/sources.list > /dev/null
apt-key adv --recv-keys --keyserver C1289A29
apt-get update
apt-get install firefox-mozilla-build
Hope You Will Enjoy......