How to extend the functionality of the opensource browser?

Question

Browsers

You need to build your browser based on chromium or mozzila, changing the name and logo, and this browser must also have some additional features.
The primary task is to parse several sites and create a page with data from them. That is, by clicking on the button in the browser, a person sees a page with a merger of several sites. After reading the documentation, I realized that the issue can be solved using Browser Extensions.
An idea came up: write your own extension, and then build a browser with it, so that when the user installs the browser, everything already works for him, as I planned.
But there was a problem, it turns out that in order to build chromium in this way, you can register extensions in the external_extensions.json file, but this only works for Linux and Mac OS, and for Windows you need to configure the registry (this is called an external extension), but this option does not suit me.
Regarding Mozilla, I did not find any information on building with default extensions. I will be glad if you tell me.
The question is: from which side to approach the extension of the browser functionality at the source code level, is it possible to write an extension of functionality without delving too deeply into the browser architecture, but by writing an extension (maybe even in C ++ language) and assemble it all into a single browser?
I really need advice on where to start and where to find information about this (documentation links are welcome).
Thanks in advance!

Answer 1

> There was an idea: write your own extension, and then build a browser with it, so that when the user installs the browser, everything already works for him, as I planned.
Eh , that's a bad idea. But, one must think, it is quite realizable.
> to compile Chromium like this ... for Windows, you need to configure the registry, but this option does not suit me.
You did not explain the reasons for this, and considering what you said above, it is difficult to come up with it.
Do you think that, minus this, Chromium out of the box is ready not to leave any other traces outside the directory outlined for it (that is, speaking in Windows jargon, “portable”, English portable)? Somehow this doesn't seem to be true.
> Regarding Mozilla, I did not find any information on assembling with default extensions.
Hand GNU IceCat look how it's done there.