This has never worked in Firefox. Fx doesn’t follow en_DK for dates & even if I set/leave my preferences @ en_US as default, I still get terrible defaults like dd/mm/yyyy & commas for decimal separators at least on my machine even if I toggle on/off the use system locale option. Is LibreWolf in a position to be the first Fx fork to support ISO 8601 date formats (with nerds rejoicing everywhere)?
& abduco can work if you only need attach/detach