I have said this over and over again.
Look back at the initial success when they were up against Internet Explorer 6. They offered users what Microsoft wouldn't. They didn't hamstring themselves with IE6 compatibility, and instead broke the rules of what a browser was supposed to be by only considering what was best for the user, and spurning harmful standards and technologies.
Where Microsoft hid extensions away in the registry, and their extensions had degenerated to the point where most of them were malicious, Firefox did the revolutionary thing of offering direct access to them by users. There's a lot wrong with Chrome, like their draconian and invasive updater that sometimes damages people's computers and can not be easily turned off.[1] Or how Chrome's extensions, while safer, are also crippled and prone to abuse by their forced updates.
Mozilla could easily leverage Google's weaknesses against them, as they did with Microsoft. Mozilla is not a for-profit platform, they can afford to make the things Google can not. They can have a powerful community driven extension architecture that empowers users to do things Google never would. So of course instead of improving and securing their old extension architecture, or replacing it with something just as powerful, they copied Chrome again.
It may be hard to believe, but including a built in popup blocker was revolutionary. It was called "irresponsible" by advertisers, and forced everyone else to change. Could you ever imagine Mozilla undermining the big players like that today? Even though bold moves like that are what got them their success in the first place. It wasn't branding or PR or any of that crap!
I think the only good thing they've done is containers. And containers are such a good thing for users that I am convinced that many in Mozilla were opposed to it and fought against it, and that it will be deprecated with a flimsy UX metrics justification. And toadies on HN will try to justify it because "why are you thinking about this so much? This is fine, stop complaining, jeez."
One of the smartest things Mozilla used to do was not try to implement bad technologies. They didn't try to re-create the quirks of the Trident engine; they didn't let Micosoft dictate the standards, so Javascript could not eject the CD tray, and you could not change the color of the scroll bars! They didn't waste effort trying to copy Microsoft's Windows-first proprietary ActiveX model. A much better compromise was the community created IETab extension, which just ran an instance of Internet Explorer in one of the tabs for when you needed it.[3] Could you imagine the Mozilla of today saying that if you wanted to watch DRM'd Netflix, you should just use Chrome? Is Firefox even technically capable of having something like Chrome tab anymore? Think of the resources that would be freed up if they stopped trying to have feature parity with Microsoft/Apple/Google.
They just need to have the courage to say "that technology is evil, we won't support it. Maybe someone will make an extension to support it". Instead, essential technologies like RSS are deprecated and left to the community to implement, while DRM and proprietary and problematic standards are given first class support.
Mozilla has made all of the wrong moves in the last 10 years. Their priorities are completely misguided. And I am certain that if they just centered the user over the big platforms, they would experience more popularity than a billion dollars in marketing and PR.
[1]: https://news.ycombinator.com/item?id=21064663
[2]: https://news.ycombinator.com/item?id=21233041
[3]: https://support.mozilla.org/en-US/questions/682975