For HTTPS. I will bet you they don't use perfect forward secrecy for TLS with SMTP traffic. Not because they don't want to , but because it likely isn't supported for a huge set of the servers they connect to. A large number SSL terminators/ accelerators that e.g Microsoft uses, don't likely support it and who knows about other stuff.

They do use PFS for TLS with SMTP traffic when the server on the other end supports it.