I was unable to get the userChrome.css method in @MC10's answer to successfully hide menu items on macOS, but it's possible to via an AutoConfig file.
// -*- mode: javascript; -*- vim: set ft=javascript:
// See https://support.mozilla.org/en-US/kb/customizing-firefox-using-autoconfig
"use strict";
(() => {
if (Services.appinfo.inSafeMode) {
return;
}
const addressPattern = new RegExp(
"^(chrome:(?!//(global/content/commonDialog)\.xhtml)|about:(?!blank))"
);
Services.obs.addObserver(subject => {
subject.addEventListener(
"DOMContentLoaded",
event => {
const document = event.originalTarget;
const window = document.defaultView;
if (!addressPattern.test(window.location.href)) {
return;
}
const hiddenMenuItems = [
"bookmarksToolbarFolderMenu",
"menu_unsortedBookmarks",
"bookmarksMenuItemsSeparator"
];
for (const id of hiddenMenuItems) {
const element = document.getElementById(id);
if (element) {
element.hidden = true;
}
}
},
{ once: true }
);
}, "chrome-document-global-created");
})();
The above example also hides the bookmarks toolbar menu item and separator. To only hide the "Other Bookmarks" folder, just use menu_unsortedBookmarks.
See this answer for additional context and installation instructions.
The browser DOM can be shown by using the Browser Toolbox or navigating to:
view-source:chrome://browser/content/browser.xhtml
To hide "Other Bookmarks" from the "Add bookmark" panel, add this to userChrome.css:
#editBMPanel_unfiledRootItem {
display: none;
}
Additional instructions for enabling userChrome.css are available here.
Note: This will not remove the "Other Bookmarks" folder from the "Manage Bookmarks" library window, or the bookmarks sidebar.