There is now a Thunderbird-kde package added to the repositories compiled with enhancement to integrate better into KDE just like the Firefox-kde package does.
Similarly, there are also thunderbird-firetray-appind and firefox-firetray-appind packages added to the repositories that now use the freedesktop.org standardized app-indicator API instead of the older xembed extension.
For more information on these new packages here are their related AUR pages:
https://aur.archlinux.org/packages/thunderbird-kde-opensuse/
https://aur.archlinux.org/packages/thunderbird-firetray-appind-git
https://aur.archlinux.org/packages/firefox-firetray-appind-git/
A big thanks goes out to the Manjaro developer Anex for compiling and packaging these for use in Manjaro (Netrunner Rolling).