FF 3.6.17 has been updated to 3.6.18; but "Help\Apply Downloaded Update Now" and "Restart FF" shows 3.6.16; and never effective after updating+restarting FF (about still show 3.6.18)
There are Admin and User (regular) profiles in Vista. FF 3.6.17 was updated to 3.6.18 in Admin profile (logged in), and "Help\Check for Update" is available there.
In User profile, FF "about" appears to be 3.6.18. But "Help\Apply Downloaded Update Now" (to 3.6.16) is keeping on (again and again) after "Apply..." is followed by "Restarting" FF.
And, in other words, "Help\Check for Update" is NOT available in User profile. "Apply Downloaded Update" and "Restart" never effect the 3.6.16 replacing 3.6.18.