I don't know why MPEG-LA hasn't sued VLC, but selective enforcement is dangerous precisely because it's opaque and unpredictable. And Firefox still has a lot more users, more usage, and a much higher profile than VLC.
It's highly questionable that the MPEG-LA considers Mozilla to already have a patent license just because we don't "sell" Firefox. You don't get a license just by meeting the terms, you have to explicitly enter into an agreement. So I suggest you prove your point by obtaining a free license from the MPEG-LA to distribute a large number of units of a product with a price of zero. Let me know when you've got that.
Beyond that, what if someone sells a product that includes Firefox? What if the MPEG-LA revises their license terms? What if the corporations that run the MPEG-LA just decide they don't like us? You'd have us bet everything on the long-term goodwill of the MPEG-LA. That would be extremely foolish.