VLC plays anything but it sounds like the files are not being handled properly. Going extreme here : uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Reinstall with Current Release Firefox 57.0 with a Full Version Installer https://www.mozilla.org/firefox/all/

Please let us know if this solved your issue or if need further assistance.

I've proceeded exactly as you suggested. Unfortunately this didn't resolve the problem. Downloaded files are still unusable. Here is the link I've just tried: https://www.arte.tv/fr/videos/050770-001-A/les-mille-et-une-turquie/

Would it be possible to return to the previous version of downloader ? With the actual one (7.0) about 30% of downloads are successful ... other - unusable.

Try this:

Users on Firefox Quantum, which normally only works with Web Extensions (a new type of plugin architecture), can enable support for legacy add-ins through about:config, search for extensions.legacy.enabled and set it to true.

See if this helps in restoring it until a new version is updated if they update it.

Thanks,

I've just set extensions.legacy.enabled to TRUE. But what would be next step ?

Pavel_47 said

Thanks, I've just set extensions.legacy.enabled to TRUE. But what would be next step ?

Well restart FF and go to your Addon extensions and see if you can still see the addon there. I haven't gotten to FF57 but my test laptop has but will update the Addon on later to see if it holds true.

Yes, Video Downloader 7.0.0 is still here. Should I uninstall it and try to install some previous version ?