I have my desktop extended across two displays. If I disable Hardware Acceleration in Firefox, entering fullscreen causes the window to be slightly offset down and right, leaving a thin band on the top and left sides of the screen where the desktop background and taskbar are visible behind the Firefox window.

From testing I have determined that this issue does not occur if I only have one monitor enabled or the monitors are mirroring the same display. I have also determined that the issue only occurs when Hardware Acceleration is disabled.