Problem here resolved (Vista64 Ultimate SP1).
I followed this thread and went into Control panel | User Accounts
to 'Turn User Account Control On or Off' and ticked the box then realised that UAC had been off already so I unticked the box again.
However while i was in User Accounts I looked at 'change your account type' and selected 'Administrator' and then restarted the computer after which download (Firefox) now OK !
I don't know why this worked because my account was 'administrator' originally, so no changes were made other than to 'toggle' the UAC on/off and 'Account Type' values.
Hopefully this may help others, there are apparantly a number of different reported solutions to this problem and this is another