We recently updated the HTML spec to match Firefox's behavior and have navigator.plugins and navigator.mimeTypes always be empty: https://github.com/whatwg/html/pull/6296

Since MimeTypeArray and PluginArray are always empty, you can dramatically simplify their IDL and backing C++ code. I thought you might appreciate a tracking bug for such cleanup work.

We may in the future remove MimeType and Plugin entirely: https://github.com/whatwg/html/issues/6317 . (The spec does not yet do that though.)

We do remove MimeType and Plugin in Bug 1682030 (a big NPAPI removal refactor, not yet landed) but we leave MimeTypeArray and PluginArray. The reason is that the array types are visible to JS so we keep them, even though there are no plugins (or adjacent mime-types) that we support so those arrays are always empty. Their contents -- MimeType and Plugin -- can't exist so we don't need them. The array classes are hard-coded to contain no elements.