I'm filing this per bug 1339081 comment 35. However, as I said in bug 1195930 comment 77, I'd like to wait 1 - 1.5 years. That's roughly Firefox 56 - 61. If a major storage version upgrade happens between FF 56 - 61, we should use the opportunity and get rid of .metadata files. The reason for waiting 1 - 1.5 years is that I'm quite paranoid that some users can launch new Firefox with an old profile and then get back to old FF. This is not very likely, but it can happen in theory. The current dual metadata files is not such a big problem in quota manager code, so we can wait a few months more.