When the browser.tabs.loadBookmarksInBackground flag is set to true, you can open bookmarks in new tabs without switching to them by middle-clicking or right-clicking and selecting 'Open in New Tab'.
However, whenever you do this in a bookmark folder, the folder closes. Meaning that if you wanted to quickly middle-click open a few tabs, you would need to re-click on the folder each time.
The suggestion is to change the behavior so that the folder does not close when opening in a new tab, similar to how this works in other browsers.