I know this reply is long overdue. i7MEDIA recently purchased mojoPortal and we're trying to get the forums caught up and get a new release out in the next few weeks.
To answer your question, Silverlight only works in Internet Explorer. Edge has never supported it and the other browsers have dropped support for it. Microsoft quit developing it a few years ago and suggested everyone using find something else.
So, it'll be removed as an option for mojoPortal Image Galleries in the future.