Full screen Flash video on sites such as youtube while continuing to use your PC - linux

Another simple fix....

emerge hexedit

sudo hexedit /usr/lib64/nsbrowser/plugins/libflashplayer.so

Hit tab to jump into the ascii panel on the right hand side, then '/' to open search and enter _NET_ACTIVE_WINDOW_ It should show you where it's found in the text, make sure you have the cursor in the right spot. Change this to "__ET_ACTIVE_WINDOW" so you've just changed the N to a _. Now save (control+X) and restart firefox.