Yes, for users who do not want to install DRM, this will not affect them. But this will affect everyone else who will simply follow the default reccomendation and end up supporting DRM.
Mozilla argue they do this because they cannot go against the trend in the market to impose DRM on HTML5 videos and thus, in order not to lose market shares because of this, they will support it.
I think this analysis is wrong, because the Firefox market is wider than just the US and the biggest video website in the world does not have DRM at all (except for the paid-for service, which I never heard of and is probably again only used in the US).
DRM are a nuisance and their costs should not be a shared burden for Mozilla and Firefox users worldwide.
Just my personal opinion
Best,