Have you verified your mouse and keyboards are working and isn't causing this to change as such or Addons causing this to happen.

Normally, Firefox for Windows uses standard zooming, where you can reset the zoom level by clicking the zoom level indicator in the address bar (the button should be present whenever the zoom level is not 100%). However, there might be some touchpad gestures or mouse magic to trigger magnifying zoom, sometimes called pinch zoom, which doesn't set that button, so you need to do the opposite gesture to reset the magnification level. But I don't know whether that affects only the current page or current tab. Does the problem persist in a new window (Ctrl+N)?