I have nothing against the Mac Application store, but that comment is nonsense. Firefox now automatically downloads updates if they are available. The MAS would be an added intermediary.
I agree, having a one stop place to get software is great. It's OK for medium to low risk software, But in the case of a Browser that we use all the time, with security bugs found almost every day, it needs constant and auto updates so it's better for both parts, the company and us the end users to get those updates outside the MAS.
Google Chrome "Stable" is one good example of auto-updates every 1 or 2 times a week. On MAS those updates would take double the time to reach the end users.
Not licensing. Cost is a factor though. Some previously free open-source apps are paid apps on the Mac App Store, because it does cost the developer to release even a free app, and they want to recoup their costs there. Others simply don't see any advantage to releasing their apps there, like Firefox and Chrome.