Tim wondered: would you recommend upgrading to a 64bit environment?
Definitely not. In fact, the solution to my own problems might be to do a clean install of a 32 bit environment. Its just not worth the aggravation. Maybe if you need to be able to address more memory. Or if you don't install all kinds of odd things that are 32 bit only - something which has given me untold grief. There don't seem to be any gains, and there is lots of aggravation.
I have narrowed down the FF .xsession-errors issue a bit further, it seems to have something to do with the user, and to be not just with FF. I created another user, started up FF, exercised it a bit, and the errors file was empty.
So another and less drastic action than doing a new clean install might be to create a new user and move all the files over. But equally, maybe the thing to do is just get it over with and move to 32 bit.
Peter