Searching for bookmark folder names in Firefox Quantum?

You can install the "Bookmark Search Plus 2" add-on:

https://addons.mozilla.org/en-US/firefox/addon/bookmark-search-plus-2/

It will add a button to toggle a bookmark toolbar (pretty much the same as the default one), where you can do your searches. Among the results, folders matching the searched word will be shown as well.

As a plus, the search panel will be splitted in two, having the upper part showing the search results, and the lower part showing the selected result in the bookmark tree. This is very convenient when you have multiple bookmarks matching the same name.