Review for Message Notes Plus by ths
Rated 5 out of 5 stars
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.
To create your own collections, you must have a Mozilla Add-ons account.