I have used Firefox on Ubuntu for several years now on at least a dozen
different machines. This is the first time I have encountered this problem. I
attempt your solution every time that I bring up Firefox, and it only works for
that session. That is why I am asking for the name of the file that Firefox uses
to store preferences. I just recalled that when I run sudo firefox from a
command line, it starts correctly. This should be telling me something, but I am
not sure what! I suspect that either I have two properties files, or that
somehow the permissions have changed.