The telemetry link in the article points to Firefox 45 data. Curiously, the error rates appear to be going up: Firefox 46's error rate is 0.10% and Firefox 47's error rate is 4.35%.
Wouldn't that make sense because FF 46 was not an ESR, 47 is the latest, and most users have been pushed to 47? More users are triggering more bugs in the parser.
Also, 45 ESR doesn't include the Rust code on Windows (I heard that's landing in 48, if a Mozillian could please confirm that), so that's a large userbase to not have included in testing.
* Firefox 45: https://telemetry.mozilla.org/new-pipeline/dist.html#!measur...
* Firefox 46: https://telemetry.mozilla.org/new-pipeline/dist.html#!measur...
* Firefox 47: https://telemetry.mozilla.org/new-pipeline/dist.html#!measur...