See glfw documentation on thread safety: http://www.glfw.org/docs/latest/intro.html#thread_safety
Basically you can render from any thread, but window event processing must happen on main thread due to limitations of some of platforms. On some platforms it will work fine if you consume events on non-main thread, but on some platforms it won’t work at all. So if your use case of glfw is to maintain cross-platform compatibility, process window events only on main thread.