I am running Linux in the form of openSuse Leap 15.0 with KDE Plasma 5.12.6 and have applied patches regularly. If I run some applications, for example Gimp (2.8.22), and ask for help, it starts Seamonkey in edit mode. Specifically, in Gimp, when I choose Help - User Manual - Basic Concepts from the menu, a Seamonkey process is opened with command
/bin/sh /usr/bin/seamonkey -edit /usr/share/gimp/2.0/help/en_GB/gimp-concepts-main-windows.html
I have looked in various configuration settings (e.g. in Settings - System Settings from the menu, then Personalisation - Applications - Default Applications - Web Browser, where Firefox is specified for http(s)), but cannot see where it says to use Seamonkey.