There are lots of ways to install the Flash Player Debugger in Ubuntu. My favourite however is to do the following:
Open Nautilus "sudo nautilus".
Drill down to /usr/lib/flashplugin-installer/
Download the debugger from Adobe.com
Open it up, take out the libflashplayer.so
Replace the version in the flashplugin-installer with the debug version
Restart firefox
Thats it