Antergos system with Qt 5.6.0, Plasma 5.6.3 and KDE Frameworks 5.21.0 with the newest updates on today (the moment of writing this post).
None of the tested by me applications are not restored after relogin plasma session/restart system. I tested: Firefox, Thunderbird and new Opera. I think all of them base on GTK2. All tested by me KF applications (konsole, kwrite, dolphin) are restored correctly (applying correct desktop number). The same issue I noticed also in KaOS system.
I’m not sure if this is ksmserver problem or integration of Plasma in Antergos. I noticed that in Mageia Cauldron (not stable, but based on the newest Qt and Plasma) only thunderbird is restored, but no KF applications). Here (Antergos) only KF applications are restored.
I made some test to be sure this is no GTK problem. Test consisted in hardcoding in ksmserverrc file properly lines that forced running of Thunderbird. Result: after login Thunderbird has been run. Unfortunately after relogin Thunderbird didn’t start. After that I checked ksmserverrc file and noticed that my entries disappeared.
For someone using openSUSE Leap it’s working
Check this https://bugs.kde.org/show_bug.cgi?id=343518