Msnim: addresses do not work
From MozillaZine Knowledge Base
msnim (MSN Instant Messenger) links do not work by default in Firefox. Instead, you receive a message "Firefox doesn't know how to open this address, because the protocol (msnim) isn't associated with any program.". This is because MSN does not register itself in Windows as the handler for msim: addresses. To resolve this issue, you must register the msnim: protocol in Windows manually.