If the dev tools are opened from a window that is very small, they are largely clipped and it's (sometimes) very difficult if not impossible to use them. Instead, if the window is below a certain size threshold, the dev tools should open in a separate window (as if the user pressed the "Show in separate window" button). This is an incredibly frustrating issue, since it means much of my debugging time on certain problems is spent resizing windows and moving the dev tools out of popups and then resizing the popups back to their original size.
Whether the toolbox is docked at the bottom, the left, or displayed in a separate window is a user preference that I don't think DevTools itself should change, even if the window is too small. Also, the mode is persisted as a preference, so users would have to do that just once. If I understand the last sentence in comment 0 correctly, it looks like you have to first resize the window in order to access the "show in separate window" button, and then resize it back. I agree this is frustrating. However it is no longer necessary. In Nightly (FF55), the button is always visible, no matter how small the toolbox is. So, if the toolbox was docked at the bottom, and the window very small, you'll see the button right away, so you will be able to click it, and then that will be persisted.