In Firefox, when you click a bookmark, it replaces the current page. To open it in a new tab, you need to press Ctrl while clicking the bookmark. I think a bookmark should open in a new tab just by clicking. I just found an extension that does exactly that.
The extension also enables you to re-open a tab that you closed but on a second thought you regret that you did. Actually, it can reopen the last 10 tabs, or more if you set it to.
Get the click2tab extension from here