The AV1 decoder is considered untrusted that it must run in the RDD process (which was created for AV1) and that there are objections against bug 1657041. AVIF blocks bug 1621548 while targeting bug 1595994 and bug 1456748. IMHO, if it doesn't run in the RDD process, it shouldn't yet be listed yet on about:preferences#experimental.
Bug 1639409 Comment 5 Edit History
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The AV1 decoder is considered untrusted that it must run in the RDD process (which was created for AV1) and that there are objections against bug 1657041. AVIF blocks bug 1621548 while targeting bug 1595994 and bug 1456748. IMHO, if it doesn't run in the RDD process, it shouldn't be listed yet on about:preferences#experimental.