HAH, i got Fullscreen to work again.....most ^^
You have to follow the Instructions in order to get Fullscreen running as far as i got...
1. Start Kirsten S19 (379)
2. Log in to Second Life and press ALT + Enter and wait until your UI and the World gets Fucked up in Fullscreen
3. Make Sure you arn´t under Water , THATS TOTALLY EXTREME IMPORTANT! - because Kirsten will crash 1000% if your under water
4. Make sure you have something to Tab to on your Desktop, IE,Firefox,WMP or something , NOW switch back to Kirsten, everything will get reloaded , and your UI should be fine now, like the rest, then go to Graphic, select your Desired Resolution (to come in next version, again....) TADA you are in Fullscreen! , but remember its experimental, i cant guarantee that it doesnt crash all the time.... and avoid switching between Desktop and Kirsten.....a known SL Problem that it will crash on some point...
Im putting the Fullscreen options in there right now again , because Resolution stucks at 800 x 600 , maybe you can change them manually in the Settings.xml´s Good Luck!