Unable to delete an unknown contact added mistakenly

Hello everyone,
I am a user of yahoo messenger. Yesterday while chatting, I mistakenly added an unknown contact to my contact list. Now, when I want to delete it, I just cannot. When I try to delete it, I get the message shown below:
'Last address book operation did not succeed due to a server error ( error code : 40402)'
What should I do?
Should I change my browser? Currently, I am using firefox version 9. The error is still there. I uninstalled it and installed version 12 of firefox. Still, the problem persists. I have windows XP and whenever I try to access Firefox, the following error occurs:
"The URL/firefox was not found on this server"
Can anybody help me to resolve this problem?
What does this error 404 mean?
Has anybody else encountered such a problem? If yes, please let me know.
Looking forward to your views.
Thanks.
