Yeah, i experience this thing to. But i mostly use alt+enter to search in a new tab. But even if i train hard to release alt key before enter, this menu pops up either. Dear developers, please, set another key to open menu.

According to my experience the only way to to this is the following:

• open Google Chrome and import Firefox bookmarks
• then copy Chrome bookmark folder at "C:\Users\User Name\AppData\Local\Google\Chrome\User Data\Default\Bookmarks"
• and insert them to "C:\Users\User Name\AppData\Roaming\Opera Software\Opera Next\Bookmarks"
  It sounds complicated, but it works in my case.
  If you have no Google Chrome installed I see no other reasonable possibilty.

Thanks albatros48, this method works with Chrome 35 to Opera 22. Easy and effective. Even the folders struture is the same. Just wonder why Opera developers didnt include this function to the latest release, when its that easy to do manually.