I tired to drag and drop my KML file onto the CesiumViewer with FireFox (version 39.0). All geometries can be displayed, however they were wrongly rendered with white color.
But, everything works fine in Google Chrome.
Thanks for the bug report and sample data. I can also reproduce this on my end as well and opened an issue in Cesium: https://github.com/AnalyticalGraphicsInc/cesium/issues/3068. My initial guess is that this is actually a Firefox WebGL implementation bug, but we’ll need to look into more to confirm (and if necessary we’ll open a bug report with Mozilla).
This KML-File can be correctly visualized in Google Earth. However, in Cesium Viewer you can see some small gaps on the roof and wall surfaces of the building.
This issue has been fixed as of Cesium 1.17, it wasn’t in CHANGES because it was unknown at the time. It was actually Cesium’s underlying geometry causing the issue.