AFAIR the alternative with the most promise involved implementing a new video codec in JS (heavily leveraging the GPU and the then-nascent asm.js) with first-class support for digital watermarking, then convincing studios that per-user watermarking was more effective than DRM. You can see the difficulty of Mozilla's position: it wasn't enough to come up with an alternative to EME, they had to come up with an alternative to DRM altogether. A tough sell, to say the least, and basically DOA because Chrome and IE were shipping EME for use with Netflix in 2012.