"WebGL warning: bindTexture: Object `tex` is already deleted." on Firefox


I’m having performance issues in Firefox (below 15 fps on an empty globe).
In chrome the performance is somewhat better (above 30fps).
When creating the Cesium Viewer with showRenderLoopErrors,
I’m getting the following:

WebGL warning: getParameter: pname: Invalid enum value 0 (Did you typo gl.SOMETHINGG and pass undefined?)
WebGL warning: generateMipmap: Tex image TEXTURE_2D level 0 is incurring lazy initialization.
[7 times]
WebGL warning: bindTexture: Object tex is already deleted.
[7 times and then:]
After reporting 32, no further warnings will be reported for this WebGL context.

On Chrome, only
Cesium.js:50 WebGL: INVALID_ENUM: getParameter: invalid parameter name
is generated.

Any ideas what could be causing this?