old firefox profile to new firefox

cf: https://www.dedoimedo.com/computers/firefox-old-profile-reuse.html

Because, firefox not allow using profile of old version, we can solve it as follows. 

1. cp old profile to firefox user directory, say, /home/chiao/snap/firefox/common/.mozilla/
2. In terminal, run:
   MOZ_ALLOW_DOWNGRADE=1 firefox -P
3. close firefox and open firefox (no need in terminal)
4. Done