I don't know if Firefox _can_ be saved.
The only reason why Firefox even rose to prominence was because it was an arbitrage opportunity back when IE6 was "the Internet" and Mozilla already had a superior browser (remnants of Navigator) that they needed to remind users existed. Firefox has been on the down swing since Google released Chrome.
When the top web browsers being used today are completely funded and highly-prioritized by the biggest tech companies in the world and are defaults in their respective platforms (Edge on Windows, Chrome on Android, Safari on macOS and iOS), and when all of them are really, really good, there really isn't room for competition.
Additionally, I think browsers as a "thing that people use heavily" are on their way out. Most people are anchoring to platforms that, at best, take advantage of webviews. For many people, "The Internet" is Google, Facebook, Instagram, Snapchat, TikTok, and $STREAMING_SERVICES, all of which are mobile apps. This, along with significant hardware and bandwidth advancements and the crypto thing, is why I think the "metaverse" is going to really take hold. But that's another HN post.
Anecdotally, the _only_ thing keeping me on Firefox is Multi-Account Containers. That's _literally_ it. I have been wanting to use Safari since forever since it's so heavily optimized for macOS and, now, the M1 CPU architecture, but the isolation guarantees that MAC provides are amazing. That's not enough to build a huge browser userbase with, though, as most people don't give two shits about tracking cookies, fingerprinting, or whatever. (I only give one shit myself, as I've started using Safari on mobile now. Firefox on iOS is just too buggy, and many web sites don't even recognize it as a valid browser, which is insane to me.)
That said, Mozilla VPN is a really good product that is not free. Maybe it's in Mozilla's interest to pivot onto web-adjacent ventures that are profitable?