Here's a Safari feature that I just ran across today that has a very slick implementation. It's called Auto-Tab, and you can turn it on inside the Bookmarks pane.
Basically what it does is take a sub-folder of your Bookmarks Bar and turns it into a button instead of a context-menu in the bar itself. Normally the sub-folders of the Bookmarks Bar are displayed with a down-arrow to the right of them, indicating that if you click-and-hold, you'll get a menu of the items in the sub-folder. Auto-tabbing turns the down-arrow into a square, and changes the functionality from a menu to a button. Pressing the button loads all of the items inside the sub-folder in tabs of the current window.
So what? Mozilla does bookmark groups, too. Indeed, but the implementation is simply less polished.
I'm now using a mix of sub-folders and auto-tabbed folders in my Bookmarks Bar.