[gentoo-user] Some precautions when updating from firefox-38 to firefox-42

[walt]

My trouble started with my update from firefox-39 to firefox-40 back
in  August 2015.  At first the update seemed absolutely benign and I
noticed nothing bad happening.

Slowly but surely firefox has been driving me crazy by doing little
things that are merely annoying.  Today firefox did something so
outrageous that finally I had to stop and fix the problem.

The big change AFAICT was the switch from sqlite to json for storing
data in your ~/.mozilla directory.  Today I noticed that dozens of
files in my ~/.mozilla directory had identical date and time stamps
from back in August, all apparently written by firefox-39.

The problem is that firefox-42 still reads *some* of those old files for
backward compatibility, but apparently not all of them.

Anyway, one obvious thing to do is start with a fresh ~/.mozilla
directory when you use firefox-42.  Or at least make a complete copy
of your existing ~/.mozilla so you can start over when you have
trouble. 

I recommend that you *not* use the profile editor from firefox-42 to
edit any old firefox profiles you may have.  You run the risk of
damaging or deleting your old profiles by doing this.  This is *not*
equivalent to backing up your ~/.mozilla directory.  Others have
already learned the hard way, including moi.