[bobtomay]

Have not seen anything that will do what you're wanting. That arrow is only for overflow when you have more bookmarks than will fit on the bar.

What you're wanting to do is eliminate the whole purpose of the Bookmarks Bar. It is designed as a place to store your frequently used bookmarks, while the more obscure are saved into the Bookmarks Menu. The bar is designed to save a click - the click of hitting 'Bookmarks' on the Menu Bar. To put all the folders of the Bookmarks Bar into a single folder eliminates the saving of that click and you might as well just move all of your bookmarks to the Bookmarks Menu and eliminate using the Bookmarks Bar altogether to get that screen real estate back.

If you are really insistent and want the Bar, but only a single folder:

Go to Bookmarks
Show All Bookmarks
now go back up to Bookmarks
Add Bookmark Folder - it will be added at the bottom of the sidebar, name it whatever you like, I'll call it 'ABC'
now highlight Bookmarks Bar at the top of the sidebar
drag 'ABC' to the right side
now drag all the other folders on the right side into 'ABC'

You will now have a single folder on your Bookmarks Bar