Longely
And they can use the main toolbar and menus, right? I'm not against folders in the main toolbar, but letting people use the bookmarks bar is a more practical solution and shouldn't be overlooked in favor of it. You can also add a button to quickly hide and unhide the bookmarks bar and then it's pretty much the same as a button folder, except you can have it permanent if you like (for a while) instead of a pop-up menu.
Edit: Since I'm feeling inspired. How about allowing multiple folder buttons, each toggling the bar for their specific content? That way you can browse with as much screen real estate as you please and only show these actions when you need them - which can be a couple of minutes, a few hours, or today but not tomorrow.
I think most people use the thumbnail gallery on the new tab page that every browser has had for a decade now, with the bookmarks bar largely not utilized or used for bookmarklets. Maybe it's time to let it be something else?