If you press Ctrl + B in Firefox you’ll get a bookmarks bar on the side that allows you to conveniently access your bookmarks. It’s like switching your bookmarks bar from the top to the side. Some people prefer it that way.
Opera doesn’t allow you that sidebar for bookmarks but you can add it with an extension. One I tried out is called V7 Bookmarks. If you use Opera just go to this Opera Add-Ons page and click on the green Add to Opera button.
Now you get a new sidebar on the left-hand side of Opera with a Star and a Plus sign.
Clicking on the star opens up the side bookmarks bar. There doesn’t seem to be a shortcut key to open it. The plus sign sends you to the Opera add-ons page.