Yettwasezmel 5 ɣef 5 n yetran

sɛur ths di Nov. 4, 2019 · aseɣwen yezgan

I was able to move my Message Notes to XNote++, another note extension for Thunderbird that also works for TB 68 and higher.
You need to locate your messagenotesplus.sqlite in the TB profile folder to extract the notes. I can add technical details of the conversion if someone is interested.