My last Firefox upgrade made it into FF 57.0b2. I attempted to remove the software, but instead I completely removed Firefox. Including the hidden files, and the files in /etc and /usr. Restarted Ubuntu. Installed Firefox from the Software center. And once again, 57.0b2 is installed again.
How do I install version 55? On the Mozilla site I can download firefox-55.0.3.tar.bz2, I don't know how to install .bz2 files.