iCloud Bookmarks Extension for Firefox is required - how to fix?

I have installed iCloud for Windows 10 and want to enable Bookmarks synchronisation. But it does not work.

First, Microsoft Edge is not supported, which is odd since it has been the default browser for Microsoft Windows for years.

So I try the Firefox extension but run into this constantly:

iCloud Bookmarks Extension for Firefox is required

I do install it, but it never seems to work.

How to fix this?