Hi @yousef,
I think from your reply, this makes the target users of Discourse are:
Mozillians who use the mailing lists for discussions in the open
I’m part of the Yammer and I think it serves a different purpose. It’s more like an internal social tool (stuff people outside of the Mozilla community would not be interested in, time-sensitive matter like launch info, etc.) There are also files and recognition features in Yammer as well.
Going back to the mailing list users, if Discourse would be the replacement, then it would make sense to get feedback from people who value it and use it.
Also, I also think positioning Discourse as an “upgrade” to mailing lists is better than “replacement” if it does present better features and will make the the target users happier.