User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Firefox/60.0 SeaMonkey/2.53.3
Steps to reproduce:
Right-clicking on a link in the browser offers "Open Link in Private Window" in the context menu. This option is not offered in the context menu when right-clicking on a link in a mail.
This option should be offered. I have attached a simple patch that enables this option. It inserts the respective code from suite/base/content/contentAreaContextOverlay.xul to suite/mailnews/content/mailWindowOverlay.xul. The base source files used to create the patch are those from SeaMonkey 2.53.3 beta 1 pre containing the 2020-04-18 patches from http://www.wg9s.com/comm-253/.