Comment 40 for bug 1203240

(In reply to comment #34)
> After discussing in private I think the best solution because of permissions
> is sticking to ~/.cache/ but ensuring deleting and purge of old files. I
> guess a new patch would be needed.

I've checked it (after adding a hack to avoid removing the file, normally external processes can't see it), and the file permissions are "600":

1605111 81156 -rw------- 1 enrique enrique 83103373 Feb 2 19:33 WebKit-Media-VPHGVY

Having this into account, there's no reason why plain /var/tmp is a bad idea, and there are several reasons why it's a good idea (as per the "cache" vs. "temporary" file discussion in previous comments).