Try removing the master password in Firefox, and then import to Opera. Make sure to backup Firefox folder first.
I've had the same issue, and it only loaded logins, not the passwords, and that works up until the version 29. After the update to Opera 30 no imported logins/passwords showed at all, until I downgraded back to version 29. and I am still on 29. so it may even be impossible to import logins/passwords to version 31. if they didn't fix that, or they even may call it a "new feature" instead of a bug.
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