Rated 5 out of 5 stars
I love this extension, thank you for porting it to TB3. :-)
For those of you who have unreadable white text on the yellow notes (e.g. due to a Thunderbird theme like PitchDark, which uses white font color), I figured out a fix.
Add the following to your userChrome.css file (in TB's profile folder (see http://kb.mozillazine.org/Profile_folder_-_Thunderbird for info on that), there in the subfolder chrome - create file and folder if necessary):
/* Xnote - set text color to black */
color: black !important;
This will set all text in the notes boxes to black.
Hope that helps.