It is possible that this issue is caused by a bug in your graphics driver if Molmil suddenly stops working on a system where it used to work before.1
Please read Using PDBj's web interface and confirm whether your browser supports WebGL. In case your browser does not support WebGL, first upgrade your browser to the latest version or use an alternative modern browser. In case your browser is supposed to support WebGL or upgrading the browser doesn't work, please upgrade your graphics driver to the latest version.
1) This issue has been reported on Windows 8.1 + Firefox 43.0.4 + NVIDIA driver 327.02