LAST_DOWNLOADED_FILE_NAME not working in Firefox or Chromium

When attempting to use !LAST_DOWNLOADED_FILE_NAME as follows:

    "Command": "onDownload",
    "Target": "${!Last_Downloaded_File_Name}-Category=${Categories}-Tags=${Tags}",
    "Value": "false"

Firefox 76 64-bit for Linux using UI.Vision RPA 5.8.8

* [warning][Line 3](moz-extension://21d3ef46-c479-49fb-9467-58437945c3be/popup.html#): onDownload - changing file names not supported by Firefox extension api yet
* [error][Line 3](moz-extension://21d3ef46-c479-49fb-9467-58437945c3be/popup.html#): Internal variable "!LAST_DOWNLOADED_FILE_NAME" not supported

Chromium Version 81.0.4044.138 (Official Build) Built on Ubuntu using UI.Vision RPA 5.8.8

[error][Line 3](chrome-extension://gcbalfbdmfieckjlnblleoemohcganoc/popup.html#): Internal variable "!LAST_DOWNLOADED_FILE_NAME" not supported

The ${!Last_Downloaded_File_Name} value is only known after the download. So at onDownload - which you need to use before the download is triggered - it can not be used.

${!Last_Downloaded_File_Name} is for example useful if you want to pass the name of the new file to an external XRUN script.

See also How to Rename a file by adding suffix to existing filename in other words how to get the downloaded filename

Thanks a million. I did not understand how the variable worked.