I'm seeing a lot of discussion over Google's use of an outdated Polymer.js version for YouTube, and its performance under Firefox.
I have some pieces of context:
1. This version of Polymer used the Shadow DOM v0 API.
2. Firefox did not implement the Shadow DOM v0 API, but Chrome did.
3. Firefox and Chrome (and Safari) now both support Shadow DOM v1.
4. This version of Polymer didn't use Shadow DOM v1, as it was developed when the Shadow DOM proposal was still a work-in-progress.