What happened with Opera was very predictable. When it comes to companies and corporations, and when their software products are proprietary, the pattern is always the same. They make something that might be good, maybe very good. Good enough to get some level of popularity. That's how they start. Over time though, the profit driven model inherent in corporations pressures them to implement questionable features - things that might generate more revenue, but are things people might tolerate at best. At some point they become more anti-features than questionable. And eventually both the company and their product devolve into garbage and we find out they've been basically an arm of the surveillance state the whole time.

Mozilla is not immune to corruption. The deal people are referring to here is that Mozilla sets the built in default search engine to whoever is the highest bidder. If I recall, there was a brief period where either Microsoft or Yahoo was going to be that company. But generally it's Google. And not everything Mozilla does with Firefox is considered good for privacy. That's why we have smaller projects like Mull - basically somebody takes Firefox, removes all the problematic parts, and adds extra security and privacy features.

But those projects have a tendency to come and go, because maintaining a complex piece of software like a browser is challenging and costly, and those projects do not generate enough revenue to be self-sustaining.

So Mozilla isn't perfect, but they are a nonprofit organization, which does provide them with a revenue model that allows them to strike a decent balance, and on the whole Firefox is a net good, and has always been one of the most important bulwarks for the free and open web. And the fact that Firefox is entirely open-source forces them to stay good.