Hi,i just purchase C2 yesterday and i have the same problem,its running at 60fps for a while then the fps drop to 8-15 and doesnt come back,firefox memory usage also goes all over the place jumping roughly 40mb memory usage.
I also tried an empty project (to be sure if it was not my fault) and i had the same fps drop,no graphic no events.
It happen between 1 to 5min after opening it on firefox 34.0.5
No issue under IE11.
(it seems to work fine on my side now, all i had to do is put the preview back on Localhost and did a 10min test without major fps drop)