Now with the obsolescence of flash firmly on the horizon, and it's usage/usefulness waning fast, it seems so be inappropriate for our notifications helper to pop up 3 mins after login with a reminder that it can be installed to 'enhance' Firefox.
In fact I've seen several comment on forums/social media recently being quite irritated with the popup.
IMO, users who did not install it, and then want it, are able to search for it fine in Muon or Discover.
Removing this involves taking out just one line of code as far as I can see
https://cgit.kde.org/kubuntu-notification-helper.git/tree/src/daemon/notificationhelpermodule.cpp#n130
Packages here: https://launchpad.net/~kubuntu-ppa/+archive/ubuntu/staging-misc/+packages?field.name_filter=notification&field.status_filter=published
Testing on a new VM seems to work ok.