O tym rozszerzeniu

Short Demo Video
right click on a paused or playing video, In the context menu select the "Copy Video Frame" entry ( Notes: if the real context menu doenst show, try holding down SHIFT while right clicking that normally gets around this issue )
To hide overlayed elements, you can use various methods, like uBO's element picker, just firefox inspector or some userscript. ) If the controls are visible in the capture, you'll have to hide them, sometimes just holding the mouse still or goint into fullsceen mode is enough, but if not, you might have to use firefox Element picker to briefly hide (display:none) them
Some additional notes & hints

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