navigator.mimeTypes properties, returning a
MimeTypeArray object respectively, have been made unenumerable with Firefox 53, following the removal of the plug-in support other than Flash in Firefox 52.
This change was originally planned for Firefox 28 to mitigate fingerprinting but cancelled due to considerable site compatibility issues.
These properties are actually still enumerable when using the
Object.getOwnPropertyNames method or simply with the spread syntax like
[...navigator.plugins], however, it makes no more sense because only Flash Player will appear if installed.