Hi, please see the youtube multi downloader website for support. That is not the proper name of the/any extension/s so no idea what to point you at other than all Extensions for Mozilla have a website with a Support site and Contact info. Follow this up with the Developer.

Please let us know if this solved your issue or if need further assistance

Best is always not to leave files in the Windows temp folder because it isn't guaranteed that you keep the files. Files in the temp folder can get cleaned up at any time by cleanup or automatically by Windows.

Did you check if it is possible to change the download location?