MetaMask only supports officially Ethereum and the Ethereum Testnets, but you can use a branch of MetaMask to use it with Ethereum classic.
(You can download from here)
See here also the code of the new development of ClassicMask.
If you don’t know the history between Ethereum and Ethereum Classic, you can read the Dexaran interview here. (I agree with Dexaran )
By MrAddon