Arthur Marsh wrote on 25/02/16 23:14: ... it turned out that I had not applied this upgrade:
[UPGRADE] kwrited:amd64 4:5.4.3-1 -> 4:5.5.4-1
I gave this a try with using org.kde.breeze again after rebooting, then set the window decoration back to Plastik and rebooted again, and started fine.
After upgrading kwrited to 4:5.5.4-1 and logging out and logging back in again and starting iceweasel with Plastik window decoration, I again had a kwin_x11 lock-up again (no usable back-trace). I set the window decoration back to org.kde.breeze, rebooted and restarted iceweasel fine, then set the window decoration back to Plastik, rebooted and restarted iceweasel fine.
So a problem still exists but it doesn't happen as often. Arthur.