In Firefox, hit ctrl + shift + o to open the library, click "import and backup" and choose "Export bookmarks to HTML".
Then, in Opera, goto the URL opera://settings/importData, choose "Bookmarks HTML file" in the drop-down and point it to the HTML file you created with Firefox.
Once imported, you can goto the URL opera://about and move the imported bookmarks and folders to where you want.
Does that way work?