Then we few who do care about those things, just become an even smaller minority.
Give them the facts and let them make up their own minds. Communicate it in a way centered around their own interest, and continually improve on this messaging. Techlore is very good at this. Accept the reality that most will simply ignore your good advice.
Mozilla may close down, but Firefox will be incredibly hard to kill completely. If nothing else, it will live on through forks (as you mentioned). A dominant fork could be expected to emerge, and might gain momentum. There are more than enough geeks to keep something like that going, even on a purely volunteer basis.
As mentioned above, we’ll just be in a smaller minority than we are even now. Some websites might not work in our browser, and hopefully we’ll have the resiliency to simply forego access to those sites, or hack around the limitations, as we always do. It’s not the end of the world.
Perhaps that’s what the “acceptance” stage of grief looks like for the death of privacy: being jaded enough not to expect any better from the average person, while accepting that at least you can create your own reality with a little bit of effort.