I completely agree that it's a pity such nice feature isn't known better among wide audience and yes, Firefox bookmarks management ("Library") UI leaves much to be desired. [2]
Just one reminder: in Firefox there is "Add a Keyword for this search..." command on any (form) input field that triggers keyword bookmark creation wizard [1], so what Chrome does automagically by visiting page with form (or using the form once?) you can do quite easily in Firefox as well, but you must find the input field, shift+f10 or click few times and pick keyword.
Also, using same keyword for different URL will (at this moment) silently "transfer" the keyword to new URL, with no warning about
[1] yet again, this wizard obscures resulting bookmarked URL with relevant `%s` part, so regular user cannot find out how this thing works. (I'm sad how hard recent browsers tend to hide whole concept of URL from users, in general. I understand it, but it's sad.)
[2] I had to `select moz_keywords.keyword, moz_places.url, moz_places.title from moz_keywords inner join moz_places on moz_places.id == moz_keywords.place_id order by keyword;` last time I wanted to see all my keywords. (And I'm trying to keep them in a single folder as well.)