Simply add a new Bookmark entry (Bar or Menu, your choice), with the desired file:///-type folder path as described in the prior hint, but then append .DS_Store to the path like so:
file:///Users/your_username/Downloads/.DS_Store
Clicking this bookmark will open the linked folder in Finder, displaying its contents. The reason we've linked to .DS_Store file here and not the folder itself is to force Safari to open the destination folder. Without the hidden invisible file at the end, Safari will reveal the location of the specified folder within its containing folder. When you link to a file within the folder, Safari will actually open the folder.
The advantage of linking to the invisible .DS_Store file is that since Safari cannot open that file type, it simply opens the containing folder in its previous view state (being invisible, .DS_Store cannot be highlighted in Finder, as would be the case if you link to a visible file).
If we link to a file that Safari can open (html, jpg, pdf, etc.), Safari will of course display that file in a browser window. Also, most (all?) folders contain a hidden .DS_Store file, unless you've stripped them out, so you can be reasonably certain one will exist.
Caveat: Unlike the previous hint, you will not get the folder's icon in your Bookmarks menu. Instead, the .DS_Store file's generic-file icon is shown. I'm not sure if you can give .DS_Store files a custom icon to remedy that.
[robg adds: This hint is mainly for those who use list or icon view. The previous hint merely selects the specified folder in Finder, but in column view mode, that means you get to see the contents of the folder in the next column. Icon and list view, though, need to point to a file within the folder to get it to open automatically.]