I’ve talked about this with @jackpot51 but commenting here so more people is aware.
Using Firefox 57/58 (unsure about earlier versions) with some privacy settings described in the issue below, I’m not able to access Mattermost anymore.
This https://github.com/mattermost/mattermost-server/issues/7996 is exactly the issue I’m hitting.
Seems like upgrading to Mattermost > v4.6.0 will fix my problem. In #7996 they are also talking about another problem when using some webrtc settings. That seems to be an issue in webrtc-adapter which is tracked here https://github.com/webrtc/adapter/issues/757