Comment 3 for bug 1290150

Have you logged out and back in again after trying this I think you need to do that for lxseisson defualt changes to take effect. I think for that to take effect to get the browser button on panel you have to click the set debian default program under the more button. The browser quick launch icon in the panel basically runs the command x-www-browser which is set by setting the debian default. Thing is on most 768 pixel high screens most of the other buttons for set debian defualt is off the bottom of the screen by bug but not for the browser supsringly. Although the debian defualt unfortanetly is system wide.

https://bugs.launchpad.net/ubuntu/+source/lxsession/+bug/1282794