Can't start backup

I'm trying to backup using adb backup org.mozilla.firefox on a Nokia 8 (with Android 8.0). I get the screen where it asks for a password. There are two texts at the bottom "Cancel" and "Backup my Data". They look identical (i.e. it looks as if I can click either) but nothing happens when I press "Backup my Data". What is going on? I also tried to enter a password but that doesn't help. Same effect with adb backup -apk org.mozilla.firefox: in both cases, only an empty backup.ab file is created. To be sure, I've also included the -f option to explicitly specify an output file name – same effect, the file stays empty. adb version 1.0.39.