How to keep Firefox v.57,58,...+ bookmark menu open so i can hold Ctrl key and click multiple bookmarks to open them?

Here is one solution that works for me:
Firefox 57+ users can set open page: about:config (from address bar) and search for: "browser.bookmarks.openInTabClosesMenu" and click to set it to "false" to enable basic "stay open" behavior now built-in to Firefox.

For older Firefox versions, one may try addon https://addons.mozilla.org/firefox/addon/stay-open-menu/