Hi   ! That's must be so annoying;   'sure hope you'll find this helpful:
'How to fix preferences that won't save':   http://mzl.la/1BAQrbZ

I read it and I still don't know how to save firefox preferences in general. Perhaps it can't be done? and I have to work hard every tiny update like I did today when upgrading from 49.0.1 to 49.0.2

Is it possible that maybe you used the   'Refresh'   feature   ?

Maybe this will help:
'Recovering important data from an old profile':
          http://mzl.la/1xKrzkF

And then there is:
'Profiles - Where Firefox stores your bookmarks, passwords and
other user data':
        http://mzl.la/1BAQULj