Actually, Jens, can you make sure the gfx.canvas.accelerated pref is set to true, not false and retest this on your Nvidia hardware with a recent Firefox? There is a chance this was a dup of bug 1907828 like Calixte suggests.
Bug 1907760 Comment 15 Edit History
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Actually, Jens, can you make sure the gfx.canvas.accelerated pref is set to true, not false and retest this on your Nvidia hardware with a recent Firefox? There is a chance this was a dup of bug 1907828 like Calixte suggests. If it works, please consider closing this bug as a WORKSFORME.