Firefox unable to update due to security permissions [apple mac]

[McLovin]

I have an old apple powerbook G4 1.25GHz running OSX 10.5.8 and it still runs great. Today firefox 3.6.6 has started giving me and "unable to update" error I've never seen....

Quote:Unable to Update

A recommended security and stability update is available, but you do not have the system permissions required to install it. Please contact your system administrator, or try again from an account that has permission to install software on this computer.

You can always get the latest version of Firefox at: http://www.firefox.com

This is completely baffling because I am using an Administrator account which is the highest privilege user account available on OS X. Ideas anyone??