A user script for Firefox, which opens bookmarks folders on the bookmarks toolbar when one hovers over them.

bookmarks-toolbar-auto-expand.uc.js

(function () {

    if (window.location.href !== "chrome://browser/content/browser.xul") {
        return;
    }

    const bookmarksToolbar = document.getElementById("PlacesToolbarItems");
    if (!(bookmarksToolbar instanceof Element)) {
        return;
    }

    const isFolder = (node) => {
        if (node instanceof Element) {
            if (node.classList.contains("bookmark-item")) {
                if (node.getAttribute("type") === "menu" && node.getAttribute("container") === "true") {
                    return true;
                }
            }
        }
    };
    
    const hoverListener = (event)  => {
        if (isFolder(event.target)) {
            Array.from(event.target.parentNode.childNodes).filter(isFolder).forEach(current => {
                current.open = false;
            });
            event.target.open = true;
        }
    };
    
    const closeListener = (event) => {
        if (isFolder(event.target)) {
            event.target.open = false;
        }
    };

    const foldersObserver = new MutationObserver(mutationsList => {
        for (const mutation of mutationsList) {
            if (mutation.type === "childList") {
                if (mutation.addedNodes) {
                    const addedFolders = Array.from(mutation.addedNodes).filter(isFolder);
                    addedFolders.forEach(folder => {
                        folder.addEventListener("mouseenter", hoverListener);
                        folder.addEventListener("mouseleave", closeListener);
                    });
                }
                if (mutation.removedNodes) {
                    const removedFolders = Array.from(mutation.removedNodes).filter(isFolder);
                    removedFolders.forEach(folder => {
                        folder.removeEventListener("mouseenter", hoverListener);
                        folder.removeEventListener("mouseleave", closeListener);
                    });
                }
            }
        }
    });

    foldersObserver.observe(bookmarksToolbar, {
        childList: true
    });

})();

other-popups-auto-expand.uc.js

// Hamburger menu
const menuButton = document.getElementById("PanelUI-menu-button");
if (menuButton instanceof Element) {
    menuButton.addEventListener("mouseenter", () => {
        menuButton.click();
    });
}

// Page actions
const pageActionsButton = document.getElementById("pageActionButton");
if (pageActionsButton instanceof Element) {
    pageActionsButton.addEventListener("mouseenter", () => {
        pageActionsButton.click();
    });
}

// Bookmarks menu
const bookmarksMenuButton = document.getElementById("bookmarks-menu-button");
if (bookmarksMenuButton instanceof Element) {
    bookmarksMenuButton.addEventListener("mouseenter", () => {
        bookmarksMenuButton.open = true;
    });
}

Thank you. Bookmarks auto-expand doesn't seem to work on FF 69.0.2 (CSS is 'unlocked' in about:config).