Yep. We have identical back buttons. :-)
OK...check your Shift and Caps Lock Keys. Shift-click opens a link in a new window, which is what it sounds like is happening.
It looks to me like your preferences have been changed. Somewhere in your menu choices you should be able to select open pages in new browser window, or same browser window. You prefer same browser window, right? Try to find that menu choice, and toggle it on and off as needed.