I did in fact use to add large padding to the menus back when it was possible, so I couse use my drawing tablet to navigate bookmarks! But alas...

I think it's a bug specific to how Firefox handles menus though. Case in point, it only does this some of the times, usually after two levels. Just a single level, and it stays open, except when the second level is too wide to fit to the space available to the right—

As I was typing this I realized that is it. It doesn't work if the new level cannot open to the right of the menu. Then, moving the mouse away slightly closes it. But now that I've moved the bookmark menu button to the left, it stays open for four or five levels deep!

Gotta get used to the new location, but good enough!!

(I know, I need to sort out my mess of a bookmarks collection. It's almost two decade old, cut me some slack!)