determine if media.decoder.recycle.enabled allows any linkability
In Fennec, media.decoder.recycle.enabled is set to true (see mobile/android/app/mobile.js; although the comment there implies that this is only enabled on nightly builds, the definition is not enclosed in an #ifdef).
It is unclear exactly what this preference does, but it seems to cause media decoders to be reused for decoding new audio or video elements. If detectable state is left inside a decoder and if reuse occurs across sites, this could mean there is a linkability risk. We should investigate.
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