This add on works just fine with version 52.8. You do have to change an option in the Config Editor to get Thunderbird to accept it. Go to... Options-Advanced-General Click Config Editor Promise to be careful Enter... extensions.strictCompatibility ...into the search box. Right click this option and Toggle it to False.

Now install the add on.

FYI: It is the add on author's responsibility to keep their software up to date. You need to ask them if they are working on it.

Hi Airmail, extensions.strictCompatibility is already toggled to False, but I still cannot add the add-on. Is there another config that needs to be changed in order to add the extension directly from thunderbird? If not, I see that the extension can be downloaded as a .xpi file. Could that file be saved and modified in order to activate the add-on? The particular add-on that is trying to be implemented is Highlighter 0.6.5 by Andre Rodier. I have left him a message to see if the add -on will be brought up to date.

Switching the preference is only necessary for TB 60. To install the add-on in TB 52, go to the Highlighter webpage with your browser (not the TB 'browser'), drag the green 'Download Now' button and drop it on Tools/Add-ons/Extensions in TB. Installation will proceed and start working after TB restarts.

https://support.mozilla.org/en-US/questions/1198300

Thank you sfhowes. It worked!