There's a problem with either this extension or iCloud.
In the settings panel for iCloud for Windows there is a section called Bookmarks.
It defaults to Internet Explorer!
There is an option to select Firefox instead but when I do (& click 'Apply') it tells me that the iCloud Bookmarks extension for Firefox is required.
I am already aware of this & have installed this Firefox iCloud Bookmarks extension previously.
Unfortunately, iCloud for Windows fails to recognise this & therefor It's not possible to set it up.