Would this auto-update if I wanted this version?

I don't want to be off the normal firefox release cycle, but I also don't want my browser to have DRM.

Why couldn't this be an install option and a preferences setting in the normal firefox version? It could even be enabled by default. Then people who care about DRM when they install have the option of disabling it, and people who care about it later once they realize the evils of DRM could disable it in their preferences. If someone installed without DRM but wants it later, the preference could download the DRM at that point instead of automatically.

edit: thanks to cpeterso below: https://support.mozilla.org/en-US/kb/enable-drm

But I still think it would be better as an installer option.

Yes, the EME-free build of Firefox 38 will auto-update to EME-free builds of future Firefox versions. The EME-free version is not a one-off build.