Well.... in short: On my Ubuntu machine (Ubuntu 22.04, Firefox 107.0.1 64bit) I get mostly errors on YouTube. Interestingly one download yesterday worked fine - but it was the only one.
The downloadable files are listed, but... as mentioned: VideoDownload Helper says: Data may be corrupt - and refuses download. Only those videos are downloadable, which - anyway - can be downloaded using "download media" from Firefox directly. And this isn't the sense of the addon, because such downloads are simple to manage without addon.
Edit:
This is strange:
ffprobe version n4.1.5-vdhcoapp Copyright (c) 2007-2020 the FFmpeg developers
built with gcc 9 (Ubuntu 9.3.0-17ubuntu1~20.04)
configuration: --arch=x86_64 --prefix=/home/mig/git/vdhcoapp/converter/src-build/linux/64/converter-build --extra-version='"vdhcoapp"' --extra-cflags='"-I/home/mig/git/vdhcoapp/converter/src-build/linux/64/deps/include"' --pkg-config=pkg-config --pkgconfigdir=/home/mig/git/vdhcoapp/converter/src-build/linux/64/deps/lib/pkgconfig --extra-libs='"-ldl"' --enable-shared --enable-gpl --enable-libmp3lame --enable-libopenjpeg --enable-libopus --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx265 --enable-libxvid --enable-libx264 --enable-libaom --enable-avresample --disable-indev=sndio --disable-outdev=sndio --disable-doc --disable-ffplay --extra-ldflags='-L/home/mig/git/vdhcoapp/converter/src-build/linux/64/deps/lib -L/home/mig/git/vdhcoapp/converter/src-build/linux/64/zlib'
WARNING: library configuration mismatch

This WARNING irritates me - any idea?