Flash fullscreen on secondary (bigger) screen doesn't work

I have a dual monitor setup, with a small (laptop) and a bigger screen. Whenever I watch a video on some streaming website, and click fullscreen, the video takes the resolution of the smaller screen (1366x768) but shows up on the big screen (1920x1080). The remaining space on the right and below the video is just grey. I, of course, want the video to be fullscreen on the bigger monitor, and not just 1366x768. Note: This is _not_ the problem where the fullscreen is always displayed on the primary monitor. For me, it doesn't matter whether the secondary monitor is primary or not and even if I switch off the smaller screen using xrandr, and only use the large monitor, the issue remains. I am not 100% sure if the video really takes 1366x768 but I guess it does. I am using openbox as window manager. Any suggestions?