First of all, I have never heard of this useful shortcut before, so thank you for mentioning it.

Unfortunately, I'm not aware of anything like it for the Library window, only the usual Alt+LeftArrow for Back and Alt+RightArrow for Forward.

Add-ons for viewing/searching bookmarks could implement it in their own windows or sidebars, but add-ons can't directly modify the Library window.

Thank you sir.

Yes, Alt+Up goes to the parent directory in any application that brings up a windows browser dialog. To me, it's such an essential shortcut that I have it mapped to my mouse middle button in my mouse controls, so for me, I can always go up to the parent directory with a middle-click.

I used to exclude Firefox from this control, because the middle button was an essential browser control for opening links in new tabs. However, over the last year or two, middle clicking links doesn't seem to work any more in Firefox, and indeed many sites have found a way to block the user from middle-clicking their links to open in a new tab, so I've configured a different mouse button to right click and press T (to "manually" open in a new tab).

I wouldn't rule out the possibility of a startup script (userChrome.js script) being able to modify the Library window, but it's a very obscure kind of programming because the Library doesn't use standard HTML/CSS. You could pose the question on this subreddit visited by more talented Firefox modifiers: https://www.reddit.com/r/FirefoxCSS/

Thanks as always, Mr Scher! :-) You're so helpful!