Meanwhile I did run into the same error: Internal error: failed to maintain the domain 9 using libxenlight error. Created a new Qubes bug report.
Might be that you are having too many VMs running.
Since I cannot reproduce this issue at the moment, please contribute to that bug report by providing the requested debug information when you run into this issue again.
Now there is a strong warning against DispVM for anonymity generally at the top of that page that will probably last until Qubes 4.0 unless one knows about these issues and works around those. ( Qubes Disposables )
And there now is a specific warning against using Firefox. ( Qubes Disposables )