I suppose an extension is easier but as browsers can be told to go through proxies, an extension isn't necessary for adblocking. A 2nd process on the machine, a change to the browser (set localhost and a port) and you're done. Someone did something similar and called it Dan's Guardian IIRC, but that's all it was.
That's how I run mine, a local (and very ancient) version of squid blocks everything. It won't be as flexible but it works.
Usually proxying can work for https connections - by letting the proxy terminate ssl. But then you really need to trust the proxy's ssl implementation, and it probably also breaks standard security ui like viewing the cert.
The SSL implementation is the least of your problems. You’d need to have a private CA on your system that creates certificates on the fly. How do you secure that proper. You’d also have problems when reporting SSL errors between the site and the proxy to the browser because from the browsers PoV, the connection looks good.
If I'm wrong about the first point, please clarify. Web/HTTP is really not my area so maybe squid receives and passes stuff through entirely as a blob, if https is used.
Edit. What dgoldstein0 wrote suggests I do misunderstand.
That's how I run mine, a local (and very ancient) version of squid blocks everything. It won't be as flexible but it works.