In 9.7 the ability to tidy old/obsolete firmware has been added to the firmware screen in the server "Files" tab.
A server administrator account
1. Navigate to the firmware tab by clicking "Files" -> "Firmware" then click Tidy Firmware
2. You can then either (1) let the GateManager decide what is obsolete, or (2) select the obsolete firmware manually. Once you are happy with your selection, (3) click "Delete Selected".
3. A confirmation dialogue will appear.
5. The obsolete Firmware will be gone.