okay now put it in the Tor Browser Bundle on Tails, how is that done?

even on Osx/Windows, how would metamask be used with the Tor route?

I don't think you've ever done this, only replied

The Tor Browser Bundle is just a tweaked version of Firefox, so Firefox extensions should work for it. https://github.com/MetaMask/metamask-extension/issues/4771 implies that it works, maybe with some bumps. I would expect that the Tor Browser Bundle works by configuring Firefox's proxy setting, which should apply to all traffic including extension traffic, but that's certainly worth double-checking.