I found that Flash Player 220.127.116.11 Beta leaks memory badly. No such issue with 18.104.22.168 Beta. Reproducible with clean browser builds (Beta and Nightly tested only in repeat testing).
I was watching Rio Olympics live streams with 22.214.171.124, RAM usage was fairly steady. With 126.96.36.199, RAM steadily increased to 100% in about 30 minutes of steady streaming, completely used up the virtual memory until Windows started to shut down the browser. Problem disappears after rolling back to 188.8.131.52 Beta.
Flash Player version 184.108.40.206 (Released: 24-Aug-2016) appeared to have resolved the memory leaks that was introduced in 220.127.116.11. All is good.