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.