The folders in the bookmarks bar will be closed immediately when opened in a new tab. In this case when I click the middle mouse button (wheel).
Maybe not related but I set the following config:
browser.tabs.loadBookmarksInBackground = true
This option should be visible in general settings in my opinion. We could use this option (or a new config) to keep bookmark folder open when clicked with middle mouse button to open in a new tab (in this case in background). Maybe we want this behavior without loadBookmarksInBackground? You can test this behavior in another known browser that starts with the letter C, which is where I come from. There the bookmarkfolder keeps open when clicked with middle mouse button, while the tab opens in background. When you right click and choose open in new tab or just left click, the bookmark folder closes. Is there already a config for achieve this behavior?
Use case: Open multiple selected links from a bookmark folder without open the folder again and again.