This is a browser extension still in development that will let you save videos from nicovideo.jp as MP4 files without leaving your browser. It's scoped specifically to the classic NicoVideo player on that domain rather than trying to cover the entire Niconico ecosystem, which should mean more reliable detection when you're watching something. You'll click the extension icon during playback, pick your resolution if multiple are available, and download straight to your machine. Not released yet and no launch date announced, but if you regularly watch content on NicoVideo and want offline copies or backups of videos before they disappear, this targets exactly that workflow once it ships.
npx skills add https://github.com/serpdownloaders/skills --skill nicovideo-downloader