h264ify-embed-fix per wsiegenthaler

This fork of the "h264ify" extension restores functionality for videos embedded in third-party sites. This is a stopgap solution until this or another fix is accepted by the official project.

See the PR for this fix on the official GitHub repository for details:
https://github.com/erkserkserks/h264ify/pull/66

Official "h264ify" extension:
https://addons.mozilla.org/en-US/firefox/addon/h264ify