The faq doesn't specify Opera as supported. A number of online reviews DO say Opera is supported by PasswordBox.
The Blog indicates Opera IS supported.
So is Opera supported by PasswordBox or not?
From the PasswordBox blog:
PasswordBox Everywhere. It’s that simple.
Did you know that you can download free PasswordBox to all your devices? Grab our award-winning mobile app for iOS & Android devices, or get the extension for Chrome, IE, Safari, Opera & Firefox. Adding PasswordBox to another device earns you a free password space – and with automatic syncing, you’ll always have your passwords when you need them.