UPDATE: new version of Video DownloadHelper addon for Quantum Firefox was not useful for me: it requires external companion app to write files (which I don't trust and don't want to install). But I found another addon: "Flash and Video Download", which does its job quite well without any assistance from external software.
Edit: Forgot to mention but if you check the add-ons site, you should be able to find an updated extension for Video DownloadHelper.