Rated 5 out of 5 stars

@Martins Lazdans:
You might have won the bounty at https://www.bountysource.com/issues/92160540-add-support-for-thunderbird-78 :-))
I've run your qnote-0.2-tb.xpi through https://www.virustotal.com/gui/file/f1c5560a7107ff9eb47f7c04b670297d9918aeb01f4aa36280e37824840ee584/detection , it's clean. I also recognized that you did not attempt the _locales yet, otherwise it looks fine.

This review is for a previous version of the add-on (2.3.1).  This user has other reviews of this add-on.