It's a long time since I last used kde. In my setup, using Mate which is loosely based on gnome, there are utilities available for Thunderbird/gnome integration. There are add-ons such as this: https://addons.mozilla.org/en-US/thunderbird/addon/gnome-integration/ I'd suggest you look for similar add-ons for kde. You may also find utilities in your distro's repositories for integration in the opposite direction. Some ideas here: https://forum.kde.org/viewtopic.php?t=96743