In Opera, goto the URL opera://bookmarks. Look under for "Imported" folders. They might be under "Bookmarks Bar" or "Other Bookmarks".
If you don't see anything, in Firefox, hit ctrl + shift + o, click "import and Backup", and choose "Export Bookmarks to HTML". Then, in Opera, goto the URL opera://settings/importData, set "Bookmarks HTML file" in the drop-down, click "choose file", and point it to the HTML file you created with Firefox.