The Archive.org Wayback Machine has a copy at https://web.archive.org/web/20230314134254if_/https://addons.mozilla.org/firefox/downloads/file/4038541/onenote_clipper-3.9.0.xpi
I haven't tried to load it to see if it will verify (i.e., whether it's a perfect copy).
According to a cached copy of the old page, it was released under the MIT license, which would make it free to modify and redistribute subject to some minor notice requirements. So it can live on if someone wants to move it forward. (There seem to be a lot of repositories on Github, but perhaps not specific to Firefox?)