I just did a clean install of 10.9 but after some tweaking Illustrator CS5 refused to start "you cant open the application Illustrator CS5 because it is not supported on this type of mac". Booted with cmd+opt+pr to reset pram but no luck. Strangely, when I created a new user that new user could start Illustrator. Anyway, I booted the first user in safe mode (shift during boot) and it could start Illustrator, and then when I booted normally it worked! So what exactly does Safe mode do? What did it change so that Illustrator then worked again in normal boot?