When I install Firefox on a Mac, I always sudo chown -R 0:0 /Applications/Firefox.app after. What, you don't? But when trying to install Flash 9, I got this:

1008:5-5000 Access Denied Error
You do not have enough access privileges for this installation.

So I tried running the Flash install as root. Same error. Then I did this: sudo chown -R drench /Applications/Firefox.app and ran the install again, which worked, and it made me sad.