this Firefox addon may help.
If not, I fear you may be out of luck.
That addon saves the FF password list to a csv file. If you can find Opera's password file you may be able to manually add the individual password details into the Opera file. But make sure you take backups first.
How do I Import my Firefox passwords into Opera? I am thinking about useing Opera but would like to inport my bookmarks and Passwords. The bookmarks I already know how to import, just need to know how to import the Passwords.