> What's the benefit for Firefox users of including StartCom?

I think there is some benefit to maintaining a healthy ecosystem of multiple competing, independently operated CAs. The benefits of including StartCom are no different from that of any other CA in that regard.

Aside from that though, not much. Although... wasn't StartCom pretty much the only CA offering free DV certs prior to Let's Encrypt? If they're planning to resume offering that or a similar service, that would certainly be a significant benefit to overall security of the web.

Yes, they were the first I was aware of. Though, not sure if I could/would trust them again.

I can't think of any benefits, except if it is hard for people in China to get a certificate and they do need another CA. It's hard to tell from here.