Try to Switch the project to Modern Experience and use the Chromium API in Input Mode.
Reference: Input Methods
Check the unattended robot resolution to match the robot machine resolution:
Check if the robot machine scale is set to 100%.
Check if the Firefox browser is updated to the latest version
Try to remove and reinstall the Firefox Browser extension.
Reference: Extension for Firefox
Also, you may take a look here: Automation Works in Attended Mode but Not in Unattended Mode