These items exist in Palemoon. Palemoon uses pre-australis UI and it is pretty expected that it has an ability to create the items you highlighted.
This feature does not exist in post australis firefox and Basilisk though! But since already existing items still work just fine, the code is there still, all that is needed is to add a way to create them.
And yes, I didn't mention it in my previous post but I having (flexible) space would be cool too.
Since basilisk meant to showcase what is possible, and these widget separators etc. are possible, I think adding an option to create them makes sense.