First thing to do is delete your browser cache. Otherwise the bugs will just sit in the cache and repeat every time the script is run.
If asked if you want to debug the script, continue, or cancel --> use cancel.
Just because a website throws a script at your browser doesn't mean you have to run it -- especially if the script is buggy.
I'd also suggest changing your homepage to one where they don't use buggy scripts -- at least until the website fixes its problem.
If the problem is with MSN's webpage, they will probably fix it eventually. But it may be a few days.
However the script that has the problem may be an advertisement rather than the MSN site itself, and you're better off blocking those because they may be infected.
If you still want to allow the script, check the link below for a possible fix. They tell you how to delete browser cache, among other things.
https://support.mozilla.org/en-US/questions/945034