The add-on seems to be working however it downloads unplayable files. I've inspected them and they start with some weird metadata (mfhd, traf, tfhd, tfdt, ttrun - MP4? MPEG TS?) and end with MP3 metadata but the data in between is _not_ mp3 audio.

I'm using Firefox 92 under Linux. The browser plays VK music files without any problems.