Having endured:-
When I opened FF I found that it wouldn't open my Hotmail emails (Hotmail will load but won't open the messages - this works just fine in IE). I also found that 'Restore previous session' was 'empty'.
Now it is running PPRuNe in reduced graphics 'text only' format without the usual coloured borders - this also applies to another bulletin-board site that I use.
It seems to be a 'memory' problem.
for two days, I have rebooted once more and the situation is now back to normal with full graphics as expected.
Hallelujah!