Do you really think there are enough principled users who will respect their stand to keep them afloat?
Most people that are smart enough to choose Firefox in the first place (even if they are less principled) aren't the type of people to ditch it along with all of its custom add-ons just so they can watch a Netflix movie. With today's cheap storage, it's trivial to install an app to play DRM streams alongside Firefox.
Besides, principled civil rights activists have been in this position many times before in the past. Often they are in the minority in the beginning and pay a price for their principled stands, but that doesn't stop them. I'm thankful people in the civil rights movement didn't submit to overwhelming peer pressure, stuck to their principles and kept up the fight... or we'd be in a much worse world today.
Firefox has a good amount of market share and it would have been bad for DRM advocates if Mozilla would have stuck to their principles. Developers would've been less compelled to implement DRM video for the sake of overall browser compatibility.
Once again, this isn't about implementation, it's about principles. If basing choices on principles was always the best, short term business decision, we'd see a lot different behavior from many corporations.
And, DRM isn't about protecting copyright. More on this here:
aren't we asking Mozilla to martyr themselves for this?
I don't think so, but standing for principles often involves taking some risks, not hedging one's bets. Forsaking one's principles is another thing entirely, however.