A few days ago JWZ had a great take on where Mozilla is today: https://www.jwz.org/blog/2024/06/mozillas-original-sin/
My reading is that jwz thinks there was a possible future where DRM is dropped because it's as useless & impractical to enforce as cryptography export restrictions had been. Mozilla could have contributed to this future by not implementing DRM, but instead supported the outcome we got: DRM is ubiquitous, browsers that don't support it are disadvantaged significantly, and an anti-DRM streaming service (similar to GOG) no longer has any real advantage over DRM-enabled services.
It is possible that no DRM in Mozilla would have resulted in the same outcome we arrived at - Mozilla gave in, so we'll never know. But what does Mozilla even exist for if it's unwilling to stick to its principles?
If DRM weren't added to Mozilla and Firefox, then they would have continued to languish in marketshare on Windows/Mac and only would have hurt open source users on Linux/FreeBSD/etc.
The long-term gains of Firefox gaining marketshare (shaking up the IE monopoly and allowing web technologies to break stagnation) were worth the short term loss of "principals" on DRM. At least, IMO.