Many people are having the same problem (including me). I suspect that this is an MSN problem, not a Firefox problem, since there have been no changes to Firefox in the last 24 hours. I suspect that MSN made a change, tested it with Internet Explorer, and said "OK, we're good to go". Of course it's possible that Firefox made a change a while ago and it just didn't bite them until now.
As for clearing cache and cookies, that's up to each individual. I don't want to get rid of my cookies. If they weren't useful I wouldn't have enabled them in the first place. It's better just to suffer with Internet Explorer until they get this fixed. I can't imagine it will be more than a day.
