Please, give the following information:
Thunderbird version: 91.1.0 (64-bit)
Operating system + version: Windows 10 Home (20H2)
Version of the add-on: 3.0.2
Hi - I thought that there used to be an option to disable graphical quoting entirely for plain text messages (i.e. just have ">" signs and different coloured text, with no quote bars). It doesn't seem to be there any more.
I have also tried what is suggested in http://kb.mozillazine.org/Quote_bars to do this outside QuoteColors.
With best wishes and thanks for all your work.