I have tried many times to install the Flash update on my MAC (OS X 10.9). It asks me to close Safari to finish, even when Safari is not on. I even restarted the computer, never opened Safari and it still asked me to close Safari.
If you use MenuTab (Facebook plugin), it will give a false "open" for Safari. It sets as a Login item by default when installed so rebooting won't help.
Applications/Utiltites>Activity Monitor. Terminate MenuTab, and proceed with the nstallation.
Retrieving data ...