I asked Mozilla and what I understood is the crashes might depend on proceeding .mp4 files as Firefox 54.0 uses the SSE2 cpu extension that my machine does not have.
So there could be two solutions
So what is the question ?
SSE2 is from...2001, your machine isn't old, it's an antique. The only option is a complete system upgrade.
Since you have not provided the information requested, I am locking this thread.
If you are still experiencing this issue, feel free to open a new thread, following the guidelines here : INFORMATION REQUIRED WHEN POSTING A QUESTION
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