I have same problem too: https://support.mozilla.org/en-US/questions/1510109

Same here, can't fix it...I have this problem only in firefox...

As a temporary workaround, could you try double pressing Win? Does it hide the taskbar?

We would also appreciate including your Troubleshooting Information.

We're looking into this. In the meantime, if you're encountering this problem, changing the setting for `widget.windows.fullscreen_marking_method` in about:config may work around the problem for you. The default is 0, but it can be set to 1, 2, or 3.

If it works for you, please let us know what OS version you're on and what setting worked for you.

Thanks