Rated 5 out of 5 stars
I installed a second copy of Thunderbird (v45.0) after downloading it from: http://ftp.mozilla.org/pub/thunderbird/releases/
Doing so does NOT remove your current version. By using the older version, I was able to install Quicktext which then works with both versions (I'm also using v52.5).
Latest quicktext works with TB52 and TB60 again.