The problem is that the API of GLFW does not support/export this at the moment.
You can either create a new window and GL context in windowed mode, or create a window and GL context in fullscreen mode. Not toggling between both modes.
Technically, though, GLFW already implements everything for it.
At least for the Windows platform, when looking into the sources win32_window.c and win32_monitor.c.
There is a function to toggle fullscreen.
And, at least on Windows, it will work to toggle between fullscreen and windowed mode without losing the GL context *if* the display mode does not change between them. That is to say, if the fullscreen window also uses the desktop display mode (same bit depths, same dimension).
With a non-fullscreen windowed you can however of course resize it arbitrarily.
So, if you want to toggle between windowed and fullscreen, you (current) always have to create two separate windows and GL contexts for that.
What are your options now?
You can use SWT which already implements desktop display mode fullscreen toggling.