Installation
1st part: Qt5 native binary
- install latest tbtrayicon_x.x.x_amd64.deb
- copy /usr/share/doc/tbtrayicon/tbtrayicon.json to your users directory ~/.mozilla/native-messaging-hosts/
- restart Thunderbird
2nd part: Thunderbird add-on
- install latest tbtrayicon-x.x.x.xpi
- enable IMAP folders to scan in add-on options
- if the icon is not visible in the KDE systray, adjust systray options to always display it
Remarks
After upgrading from an earlier version to Thunderbird 68.2.2 the add-on did not work because of the query function raising an error.
I had to delete and re-create the Thunderbird account for my IMAP account.
This bug seems to be fixed in the latest Thunderbird version 68.3.1.