Try to export the Bookmarks in Opera to an HTML file and import that file in Firefox.

See also http://kb.mozillazine.org/Import_bookmarks ("Import from another browser" and "Import from file")

From Opera FILE>IMPORT AND EXPORT>EXPORT BOOKMARKS AS HTML Choose a suitable location and name for the file From Firefox BOOKMARKS>ORGANISE BOOKMARKS>IMPORT AND BACKUP>IMPORT HTML This opens the import wizard. Select From an HTML file NEXT and navigate and open the Opera file you have just saved. Job done.