Keith,
After much searching I think I have found the problem....
I thought I was having a bit of a senior moment, as when I modified "general.useragent.vendor" from Ubuntu to Firefox and then restarted the browser and checked, it had been reset back to Ubuntu

However it turns out that this is something to do with the Firefox add-on: "User Agent Switcher" that I had installed and setup to try and fix the problem in the first place

.....So I removed it and re-modified the "general.useragent.vendor" back to Firefox like you said and hey presto, Hotmail now seems to work OK (fingers crossed).....Thanks, your a gem.
One down two to go...unless I bu99er something else up along the way

Many thanks,
Lightfoot.