Your machine won't take it.
Pre-Installed MacOS: X 10.4.9 (8P4037) Maximum MacOS: X 10.7.5*
Mac Pro (8-core) MacPro2,1 MP21.007F.B06 (EFI 1.2) 1.15f3 (SMC 1.1)
Most firmware updates are automatically installed when you update or upgrade OS X. Some firmware updates are also available as downloads you can install manually. If your Mac needs a firmware update and it isn't installed automatically, check to see if a manual updater is listed below.
If you don't see a firmware version listed in this article, it means that either a firmware update is only available as an automatic update, or no firmware update is needed.