Search for "HLS Downloader Userscript" (specifically "M3U8 Catcher"). Once installed, when you play a private video on ThisVid, a small download button appears in the top left corner of the video player.
For years, the standard advice for downloading from ThisVid has been clunky: screen recorders with terrible audio, sketchy browser extensions that steal your data, or copy-pasting links into low-quality "universal" downloaders that fail 99% of the time. thisvid private video downloader better
Downloaders don't stitch fragments; FFmpeg does. Install it and add it to your System PATH. thisvid private video downloader better