For Slack's primary use-case of intra-organization communications, this is completely useless and wouldn't work anyway (if you block and hide messages from a team member, how are you going to collaborate?). If there is abuse, it should be up to management/HR to step in and address this issue.
Mozilla should really mind its own business - Firefox marketshare is dwindling and so are the reasons to pick it over its competitors (and they don't even focus on the few reasons that do remain!).