I understood tenure to more or less mean "We've found your contributions valuable enough to date that we are now ensuring that you can never be fired for having

I understood tenure to more or less mean "We've found your contributions valuable enough to date that we are now ensuring that you can never be fired for having the 'wrong' opinions, thus guaranteeing your academic freedom." Whether that academic freedom is intended to enable teaching or further research probably varies by institution and subject/department, and maybe individually as well.