The writing has been on the wall for a while - as soon as Mozilla realized that Google was going to renege on their promise to remove H.264, Firefox was bound to include it (since H.264 wasn't going anywhere). As soon as the W3C enshrined DRM in the HTML standard, it was a matter of when, not if, Firefox would be forced to, assuming the media industry adopted it (which they will).
The only alternative would be for Firefox to hold out and refuse to adopt it, watching their userbase shift away to all the other browsers that did support it.
[0] http://blogs.computerworlduk.com/open-enterprise/2013/11/bre...
Meanwhile users abandoning Firefox for not supporting DRM in the browser is not at all a foregone conclusion. More likely is years of content providers forced to support other browsers, giving more leverage to those seeking other solutions.
Eich helped found Mozilla back when it was just contributions to Netscape, and then helped break off as a fully-fledged project. My guess is that he understood the loss here. On the other hand, Gal wrote PDF.js which replaced the proprietary PDF reader, so you'd expect him to get it, too.