I have followed all 'troubleshooting' hints and am still unable to complete FlashPlayer installation. Have turned off firewall and plug-ins are allowed. I have OS X Maverick and have uninstalled previous version of FlashPlayer. Still no luck installing new FP version past 55% on download progress bar.
Download the FULL offline installer: Flash Player 12 (Mac OS X)
Save it and quit your browser.
Mount the DMG (double click it)
DO NOT Run the installer.
Right click the installer and choose "Show Package Contents"
Go to: Contents/Resources and double click the Adobe Flash Player.pkg file
The installer will open, ask for your Admin password, and it will install.
BEFORE opening any Flash content in Safari, open Safari>Preferences, and check the Security tab to make sure "Allow plugins" is checked.