It appears that mozilla creates a subdirectory in the user's home directory with the user's name, eg /home/fred/fred. I ran <dpkg --purge mozilla> which according to my understanding of the manpages should eliminate all traces of its existence - it didn't say that exactly, but this seemed the most radical option.
However, the directories are still there, so: Should I have done it another way? Has mozilla left any other droppings on my system? If there is no other way to clean up, shouldn't there be? TIA, David