Virtual Desktops should be familiar to anyone who is/has used an X window manager.

Effectively virtual desktops allow you to ararbitrarily aggregate windows to reduce window clutter.

e.g. I have desktops for Mail, Browsers, Office Applications, Photo Manipulation, Local and Remote terminal windows, and Code Development.

All of which are acaccessibleia hot keys and all of which have the appropriate windows directed to them when new windows are created.

I have recently found an interaction bug with Virtual Desktop and Firefox/Thunderbird. Fortuneatly I have also found a work around.
The problem presents as Firefox/Thunderbird (F/T) occasionally not accepting input from the keyboard, although they do respond to mouse events.

The workaround:

If F/T are not responding to keyboard input, switch to a different desktop and back. This has fixed the issue every time, since I have started using it.