I have already installed firefox 52.0.1 (64), trying to install 50.1.0 (64)
sudo apt-get install firefox=50.1.0
answer
E: «firefox» version «50.1.0» is not found.
More information:
$ apt-cache show firefox | grep Version
Version: 52.0.2+build1-0ubuntu0.16.04.1
Version: 52.0.1+build2-0ubuntu0.16.04.1
Version: 45.0.2+build1-0ubuntu1
