What technical reason is there for Firefox to dynamically download these libraries instead of bundling them at built time? It leads to bugs such as this one.
Licensing. Cisco has an all you can eat distribution license for the codec, so Firefox downloads from Cisco to piggyback on the Cisco license. If Firefox bundled it Mozilla would need a distribution license, and that's expensive.
Pretty sure its up to Debian to repackage and modify software to meet their standards. Not for app developers to somehow comply with every bizarre packaging policy each Linux distribution cooks up.
Because if the user doesn't use netflix, you don't have to bother them with widevine or EME, it's not even installed. It only gets installed if necessary and Mozilla can ship the occassional update without having to wait for distro maintainers, preventing Netflix from not working.
