Browser crashes when "Save frames" clicked while "preview" selected

[JapanYoshi]

When I have the "preview" checkbox ticked (as I often do, checking if the shader looks correct at the specified resolution and framerate), pressing "Save frames" makes the browser hang from using lots of resources and resulting in either crashing the browser or entirely locking up the computer.

It would be great if pressing "Save frames" undid whatever the preview checkbox does, so that I could save frames directly after previewing without the risk of accidentally locking up my computer.

Firefox version. Windows 10.