[openbox] firefox >40.0.3 hangs when started from a <keybind>

[Louis-David Mitterrand]

Hi,

After upgrading iceweasel (on debian/unstable) beyond 40.0.3 it hangs
indefinitely after displaying its main window when started with that
keybind:

<keybind key="W-f">
  <action name="Execute">
	<execute>/usr/bin/iceweasel</execute>
  </action>
</keybind>

However it works fine when started from ~/.xession or from the command
line of a terminal.

Any idea?