Yeah I heard the firing was just a false rumor, and that he's just on "vacation". It's also not been confirmed that a mission 51 was even fully confirmed as a definite part of the game. It sounds to me like it was just something they originally planned, but wasn't found to be suitable for the game for some reason.
There's also been rumors there will be a MGS6, possibly with Kojima involved.
I think mission 51 is based on that secret mission you have to unlock while doing certain things through out the game. I hear it gives you the bandana for unlimited ammo, not sure if its true, but i did hear about a secret mission.