This looks at first glance like some GStreamer element(s) could not be created, such as appsink, maybe others too; possibly because the required GStreamer plugins are not installed. Combined with insufficient error handling.
I note that this bug was reported against an iceweasel version that uses the old and unmaintained GStreamer 0.10.x. The current iceweasel version uses GStreamer 1.x, and I can't reproduce this problem with 38.2.0esr-1 either.
