Uninstall the old Flash plugin versions > Troubleshoot Flash Player | Mac OS
And turn OFF Extensions in Sarfari > Prerferences > Extensions. Quit and relaunch Safari to test.
And check to see if Safari is running in 32 bit mode. Right or control click the Safari icon in your Applications folder then click Get Info. If the box next to: Open in 32 bit mode is selected, deselect, quit then relaunch Safari.