Thanks, that's closer to what I was looking for. I'm still a bit unclear on what exactly the root cause of the bug is. A flawed plugin API (PlayPreview), flawed usage of that API, a bug in the implementation of that API, a combination of these or something else. From your link it looks like the plugin now does not use the API anymore, but the API still exists. But the API implementation seems to have required fixes as well. [1] [2]
[1] https://github.com/mozilla/gecko-dev/commit/2e67509c8a3cefe8...
[2] https://github.com/mozilla/gecko-dev/commit/8b7f7bde8f21bd71...