Thanks to the fix in the comments at diarizing.com, if Pidgin won’t connect because of a certificate error, then:
- Delete all msn/live/hotmail certificates in the folder ~.purple/certificates/x509/tls_peers
- Go to https://omega.contacts.msn.com using firefox
- Right click > view page INFO > Security Tab > View Certificate Button > Details Tab > Export Button
- Save file to ~.purple/certificates/x509/tls_peers/omega.contacts.msn.com
- Reconnect
