I saw the same problem. I was doing test motion renders in Preview mode and everything was fine. When I tried making animations using Firefly it would sit there for hours, then crash. I finally tried rendering as separate image files and it went much better. It would still occasionally crash in the middle, but at least then you have some image files instead of an incomplete Quicktime file. I could then restart the animation at the first missing image. So one interpretation to "try rendering in a separate process" could be: render animations as separate images, but do it in small batches". For example, a 6-second clip at 24 frames/sec would be 144 image files. You could do images 1-72 first, then do a separate rendering for images 73-144.