This is a weird one. Ubuntu 16.04. I have firefox 57+ installed in the usual way. Profile in my home dir, etc. But I needed to install 56 to use some add-ons that don't work in 57 yet. So I did something I've done 100's of times before. Downloaded 56 for linux. Unzipped it to ~/firefox56 and made a directory ~/firefox56-profile. Then I started it with ./firefox -P, created a new profile, and chose the ~/firefox56-profilel folder to store it in. Then I launched firefox56 from the profile manager, disabled automatic updates, verified it said it was firefox56, installed an add-on, and was prompted to reboot firefox. When it restarted, it was version 57. I assumed, well maybe the restart button that the add-on displayed just issued 'firefox' command in my regular PATH. So I went back to ~/firefox56 to manually run ./firefox, except the entire folder was empty... I've done this twice now, and each time I run firefox 56 for the second time, the installation folder is empty of all files.