Rated 5 out of 5 stars

Some hints and observations.
@GetSat (2) You can add a new tag (e.g. 'XNote') using the tag manager (on the main menu). Then "Use tag [XNote]" in the Add-On settings for QNote, don't forget to [Save].
@GetSat (3) Martins (developer) says: "Popup positions currently can't be set due to API limitations"
@GetSat Re "Import old XNotes and to export QNotes": QNote uses the same file format as XNote++ does. If you edit the Add-On settings / Storage: (o) Local folder e.g. /home/<user>/.thunderbird/12345678.default-release/QNote you will see .xnote files in this folder, just like under XNote++. You may also copy your existing xnote files
into the folder and you will revive your notes under QNote.
@Brother Rabbit: using Add-On settings / Storage: (o) Local folder e.g. /home/<user>/.thunderbird/12345678.default-release/QNote is good to receive all your existing .xnote (XNote++) files. After that you will see all your existing notes appear in QNote.

This review is for a previous version of the add-on (0.4).  This user has other reviews of this add-on.