I'm not sure you can import directly into the Android version of Firefox due to security limitation on the device. If you can get your Opera bookmarks into Firefox on the desktop, you can use Firefox Sync to move them to your mobile. Does this thread help? [https://support.mozilla.org/en-US/questions/923976 how can I sync bookmarks between my opera mobile browser and my new Firefox for android browser ? | Firefox Support Forum | Firefox Help]