Rated 5 out of 5 stars
Very useful add-on, I would say essential, extremely simple to use but filling an important gap of Thunderbird new versions . I have just installed it and tested on Thunderbird 78.5.1. x64 with Windows 10 20H2. It makes the highlighted text well visible also when receiving the email on my phone (Android 10 - Outlook and Gmail).
I saw no specific command to cancel an highlighting done; just an 'undo' however does the job.
New unmark function in version 3.0: hold "Shift" + click "Mark!"-Button to set background of the selection to white.