"I don't care to live in strategically lost situatios like this, so I think the conversation should be about Firefox extensions."
Why would the conversation not be about editing the Firefox source code to add or remove "features" to meet one's personal needs.
What is the point of "open source" if, to use the term from the submission title, the software is effectively un-"hackable".
There is no small amount of "attack surface", and many unneeded "features", that could be removed from Firefox to someone's benefit, maybe it's only one user,^0 but but that will effectively never happen. Why. It is open source so anyone should be able to audit the code and change it to their liking.
0. To be clear, I am not commenting about "most users" or the majority of users or whatever. I am referring to the small class of users who are explicitly dissatisfied.
In 1995, there were numerous non-commercial browsers. Netscape, the source of Mozilla, was one of the few attempting to commercialise.
https://www.w3.org/Clients.html
There is nothing wrong with having "all-in-one" programs. As long as other "not-all-in-one" programs also exist as alternatives.
Arguably, the aim of the "all-in-one" program may be to obviate the existence of other programs, namely smaller, simpler ones.
Those pushing gigantic web browsers might assume and argue, e.g., that it is inconvenient to have different programs for different tasks. This could be true. For some users. However it is also true that small programs can be made to work with each other. UNIX is the example. Over thirty years of continual growth. The companies behind the giant browsers probably could not survive without it. There is choice.
Large "all-in-one" programs and small ones like UNIX utilities can co-exist. The two are not mutually exclusive.
Personally, I prefer not to use a giant browser to make HTTP requests on the open internet. It is overkill and there is a profound lack of user control. (Hence "solutions" like "sandboxing", and an ever-incresing number of Band-Aids that serve only to add more needless complexity. The companies releasing these giant "all-in-one" programs are funded by advertising. Enough said.) For me the "modern" browser is more useful as an image viewer and media player.
It is possible to "browse" the web without advertising, tracking or other annoyances, I do it every day,^1 but not with one of these giant advertising-supported "all-in-one" programs like the "modern" web browser. It is a losing battle to try. No amount of "extensions" can change the balance of power over those giant programs.
Despite that these "browsers" are "open source", dissatisfied users who know how to program are not editing the source code to remove the bad bits. Instead they helplessly complain in forums like HN.
1. I am not a typical user. (Though I might be in 1995.) I prefer text over graphics. I like to read without distraction. Because text is easy for the user to manipulate, it seems to have a defense against advertising that is not available with graphics. For example, if text ads were inserted into response bodies, I can easily filter them out.