Are you asking about the https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/ Video Download Helper extension? Also, it would help if you posted in English.

If this issue is with Youtube then there is a Beta version of the extension to try https://github.com/aclap-dev/video-downloadhelper/discussions/1074#discussioncomment-11914403

Youtube tends to make changes (on purpose) so extensions like these may not work fully or at all and the extension then needs to be updated by the authors.

Si ce problème concerne YouTube, il existe une version bêta de l'extension à essayer https://github.com/aclap-dev/video-downloadhelper/discussions/1074#discussioncomment-11914403

Youtube a tendance à faire des modifications (volontairement), de sorte que des extensions comme celles-ci peuvent ne pas fonctionner complètement ou pas du tout et l'extension doit alors être mise à jour par les auteurs.

https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/versions/ shows this: Latest version Version 9.3.0.1 Released Jan 22, 2025 - 1.52 MB Works with firefox 115.0 and later - Fix youtube failure

So if you're using an older version, try updating to the latest version.