Safari not working anymore

Question

Since upgrading my old Macpro powerbook to 6.0.2 (using Lion) Safari will not open. Instead I get the message that this version of Safari requires Mountain Lion.
The upgrade information said it would work, and it works on my desktop (with the same configuration).
The upgrade should have not been permitted if it wasn't compatible.
I've tried reverting back, but no dice.
What should I do?

Answer 1

Back up all data.
Boot into Recovery by holding down the key combination command-R at the startup chime. Release the keys when you see a gray screen with a spinning dial.
Note: You need an always-on Ethernet or Wi-Fi connection to the Internet to use Recovery. It won’t work with USB or PPPoE modems, or with proxy servers, or with networks that require a certificate for authentication.
When the OS X Utilities screen appears, follow the prompts to reinstall the OS. You don't need to erase the boot volume, and you won't need your backup unless something goes wrong. If your Mac was upgraded from an older version of OS X, you’ll need the Apple ID and password you used to upgrade, so make a note of those before you begin.