I interned at Mozilla too; my experience there was just as phenomenal. Free $3000 laptop, random games, free food, free apartment etc. However, I think the thing that I cherished the most were the people themselves, they were committed to their mission of making the web a better place. This is especially true of the Metrics team. So many of the discussions regarding privacy and the amount of information I could analyze seemed frustrating then, yet, it is only after a couple of years in industry that I realize the true value of respecting those who trust their data with you and using it to do greater good.
As someone who's been around the web platform and standards orgs for a number of years (on browser-side stuff, since 2006; working at Opera 2009–2013), I'd like to say that I think almost everyone working on browser engines (be it at MS, at Apple, at Google, at Mozilla, formally at Opera — and hopefully again in future) really care, quite deeply, about making the web a better place. From my viewpoint, what differs most is the views of those higher up in the organizations.
Certainly, the amount of resources people put behind stuff varies (Mozilla obviously cares deeply, to the extent of it being their foremost mission; Opera has historically cared about it deeply — it used to be a point that a disproportionate amount of specs were edited by Opera employee — and it is part of their "vision"), but at the end of the day, as an intern you're likely to be working with those directly alongside you almost exclusively.
If you care about working somewhere where people are truly passionate about what they're doing — go for any browser vendor working on an engine (I say this as to exclude Opera — while there are some contributions flowing from there, they are relatively sparse in number — though I wouldn't be surprised for this to change back to them investing heavily in their browser engine as their Blink-based products enter the market). Yes, in many ways Mozilla are probably the best (as there's no question that this view goes all the way to the top), but don't rule out the others.
