Same issue here. March 20th.
What is the name of the .dmg you are using?
Can you go into Activity Monitor and verify that Safari is closed? I'd also give this installer a try if you haven't already.
I am having the same problem with this new update,
I am running os x 10.8.4 (12e55)
safari version 6.0.5 (8536.30.1)
I am having the same problem with this new update
Then why not follow Chris' instructions in the post above?
This works 99.999% of the time.
Download the offline installer here: http://fpdownload.macromedia.com/get/flashplayer/current/licensing/mac/install_flash_playe r_11_osx.dmg
If it won't download directly, right click and "Save as"
Go to: Mac HD/Library/Internet Plugins
Trash the Flash Player.plugin file from there
Go to: Mac HD/Library/Application Support/Adobe
Trash the Flash Player folder (the whole folder)
Empty the trash
Mount the DMG and run the installer.
BEFORE opening any Flash content in Safari, open Safari>Preferences, and check the Security tab to make sure "Allow all other plugins" is checked.
Safari was closed for me yet I still got the please close Safari message; However, within Activity Monitor, I could see that "Flash Player (ShorTel Communicator Internet plug-in)" was running. After closing ShorTel Communicator the message cleared and the install was able to finish. I think that the installer was unable to detect the true source of the running Flash player, or that Shortel uses Safari as a route to Flash.
The solution for me was similar to the solution posted by gwd-oc. I opened Activity Monitor and searched for "flash". I found a process named "Flash Player (Skype Internal)". After killing that process, the Flash Installer no longer said Safari was running, and I was able to complete the installation. I presume exiting Skype would have also done the trick. Apparently, what the installer is detecting as Safari is actually a Flash plugin running in some app.
click the apple icon in the top left of your mac. click 'force quit'. click on the programmes you need to quit in the box that appears and click 'force quit'. now install abode flash player.
Thanks for the help gents... I had the same trouble. It even failed when force quitting safari. I noticed that other programs (i.e.. pages, numbers, notes, ect. ) were running. I quit everything and clicked the Adobe installer icon copied to desktop. It finally completed..