My experience has been that the Library window has never automatically closed itself, although a feature like that may have been part of an Add-on / Extension for Firefox, that I never used or needed. I really never use the Library for much of anything, since the Library was first added to Firefox with Firefox 3.0. I used the {Ctrl + B} keyboard command to open the Bookmarks Sidebar, starting with the first version of Firefox I used back in 2002. Worked just like I was used to with IE 5.5 in those days. Now-a-days that is an automatic brain / left-hand action for me. (little finger and index finger)

And the same action to close the sidebar; although I did use a number of different extensions to do the closure automatically after opening one bookmark from time to time over the last 18 years.

OH, I don't think that the Library window being displayed in a separate "window" is going to be around too much longer. Many separate windows like that have been eliminated and replaced with newer "floating" displays; I forget the term right now , but might be "door hanger". Like a "floating" display (using CSS) over the open Tab interface.