Are you using the same size of window in chrome and firefox? And do you have any sidebars on display in chrome that you don't have in firefox?
When working normally, the switch from desktop to mobile view happens automatically depending on the width of the window -- in firefox, if you progressively reduce the window width you will see that happen -- exactly when depends on the size of your screen, the default font size, whether you've used control with mouse scroll wheel to zoom in or out and whether there's any sidebars to reduce the available window width.
