I've been working, in various forms, on Mozilla for 17 years now. I've joined the project as a volunteer back in 2000, and ended up working full time since 2010.
I worked in multiple roles during that time, mostly as an engineer, but spent some time working on contributor relationships, contributor tech and some market analysis even.
I've worked remotely for most of that time from Europe, but am working from San Francisco for almost 4 years now.
My personal experience has been very positive. I'm working full time on projects that impact the shape of the Web, I have good work-life balance and incredible number of highly challenging problems to solve.
I've been with Mozilla since they were a 40 person startup, through the crazy days of releasing Firefox 1.0, sudden growth during Firefox OS days (I spent 2 years working on it!), and now I work on Firefox 57 and beyond. It's been a great, if bumpy, ride and I can't stop enjoying it :)
I've seen bad culture fits, I've seen clusters of bad management and projects that "felt weird". People who got into Mozilla through those usually ended up leaving, but I'd say it's a vast minority of cases.
And wrt. "sinkin ship" - It depends on us. Working for a non-profit has some downsides. We need to stay open, inclusive, some things happen slower, we get much more "negative feedback" from people who feel more entitled to judge our work etc. But it also comes with some huge benefits. I never optimized anything at Mozilla for profit, our leadership sees money as a tool, and we can focus on what we solve with the tool. Some of my long-standing projects are way too "altruistic" to ever exist at Google, Apple or Facebook, because they don't impact the revenue stream. We can and use the Mozilla Manifesto to reason about decisions all the time, and thanks to all of that - the Web community really supports us.
So, if we can get amazing products, we may not have the marketing budget of big companies, but we have the support of the global community. And releasing amazing products is something that depends on us, not any external factor.
Right now we're pushing for Firefox 57 and I do believe we're on the right track to succeed (yes, I disagree with Andreas :)).
My take is that if you have the "NGO" muscle in you, enjoy spending a lot of time on big problems, like the open-source community model of work (which we drift towards with most projects, although there are pockets that are slow to open up) and are not money-driven (not because they pay bad, just because you can get better money as a hired gun at some corpo ;)), Mozilla is worth giving a try :)