I believe if you are on 4.1.x and If the device is in active/passive and you can login to the other device. You can create a super user on that device and sync it to the one which you have forgot the password for. Then login to the firewall with the new account and change the password for admin.
Hope this helps.
I am not sure on the version it might be 5.0.x where the administrator accounts do sync however the admin account might not get sync'd. As far as i remember i think it was introduced in 4.1.x version.
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