This extremely useful style which added scrollbars to all long Bookmarks folders was broken by the Fx155.0 update... unsure why?
The style was originally posted by OP nollinvoyd in his March 2023 r/FirefoxCSS topic 'Scrollbars for all bookmarks folders' and had worked flawlessly since up to and including the Fx154 stable release.
/*** Scrollbar for long Bookmark menu "popups" ***/
[placespopup="true"] { --allow-scroll-bar: auto; }
scrollbox[part="scrollbox"] { overflow-y: var(--allow-scroll-bar, inherit) !important; }
Unsure what has changed in Fx155... possibly the variable --allow-scroll-bar has been changed or removed? Unable to find any related bug after searching the huge 'Target Milestone 155' Bug list.