Surely that hasn't trashed your installation ... only M$ updates do that?
Don't I know it!! No, everything is there ... bookmarks etc. it just doesn't seem to actually start.
... try to back up your profile folder before you do!
What is a profile folder and how do I back it up please? I tried the ctrl/alt/del thing but it shows me that Firefox is not running, so that's not the prob.
UPDATE ...
Okay ... all sorted.
It occurred to me that programmes like Firefox often get updates as a matter of routine, so I thought maybe one of these updates was "disagreeing" with my firewall. I uninstalled the firewall AND Firefox then re-installed Firefox which then worked fine (without the firewall). I then re-installed the firewall AFTER Firefox and everything is now okay. We'll see if it stays that way! Thanks for your help though ... much appreciated.