In Firefox I have many sub-bookmarks in the bookmark panel on the left of the Firefox screen. Frequently many of the sub-menus stay open or open when a new web page is opened. I know there is a quick way to collapse all the sub-menus without stepping through each one to do it.. How do I do it?

The only way I know is to install the extension "All-in-one Sidebar". That extension has a zillion options including one that allows you to keep only one subtree open (by closing the others each time you click the plus sign)
But... so much for that, I'm looking for a simpler way to do it.