Gstreamer crashes when playing any video with Totem, Firefox or Nautilus

For months, I have had crash reports (under Nautilus) or Totem or Firefox crashing when accessing a video. This seems to be due to Gstreamer. To be noted that I can watch Youtube video under Firefox, but visiting some website makes the application crash (including the ubuntu.bq.com one...).

if launched from the terminal, the error message is:

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(gst-play-1.0:23698): GStreamer-CRITICAL **: gst_structure_new_empty: assertion 'gst_structure_validate_name (name)' failed
Redistribute latency...
Segmentation fault (core dumped)
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I installed all the plugins possible and my computer is up to date (Ubuntu 14.04).

Cheers,
Pierre