The solution I plan on implementing:
* Add get_process_id() to native_events library.
* Use the process_id in the Firefox extension to write switch_window_started.<pid>
* From the ignore_nofocus library, read file switch_window_started.<pid> (Getting the
pid from the native code is trivial).

Andreas found a simpler solution - in-process function call.  He's working on implementing
it now.

Will this work in the case where multiple processes are running tests using the same
display? Surely only one window at a time can have focus....

As far as the window manager is concerned, only one window has focus. However, none
of the  Firefox instances will realize it's lost the focus.

Fixed in r14011.