Hello, thank you for the comments.
Indeed, this is facing 2 limitation from Firefox WebExtensions API =

1) & 2) Tags, Keywords and Description are not made available to add-ons.
See Bugzilla https://bugzilla.mozilla.org/show_bug.cgi?id=1225916
and dev documentation https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/bookmarks/BookmarkTreeNode
No such field ... => no way to get that today :-( (note: the bug is only 2 years old, there is hope ..)

3) Events occurring out of the frame (sidebar) where the add-on is running are not dispatched to the add-on by FF.
So when you click outside the sidebar, like on an open browser page, the add-on is not informed and does not get any event. Same for keys .. => nothing can happen as the add-on simply doesn't know.

However, when you click anywhere else inside the sidebar, the context menu gets closed.