As far as I can tell yes it's web specific and appears to be a source texture size issue with Firefox
If you are the 2bd person it's happening to in hoping you can confirm that for me!
Hello, I have the same black rectangle 'bug' on firefox. Firefox console indicates the error below, which may be relevant.
WARNING: 2D MSAA is not yet supported for GLES3.
And also the following warning, which may indeed be linked with your texture size
WebGL warning: texImage: Requested size at this level is unsupported.