[openbox] sometimes shortcut keys don't work when firefox started

[Xiaoguang Ren]

Hello everyone,
lately it happens everyday 1 or 2 times, that those shortcut keys like
alt+tab or which i defined don't work when firefox started. And when
closed firefox, shortcut keys work well again.
I don't know if it's firefox's problem or openbox's problem. have
already submit this problem to firefox.
any idea to this problem? thx

System: archlinux i686 2.6.31
openbox: 3.4.9
firefox: shiretoko 3.5.6