Yes, Mozilla is keeping the whole functionality. You can MITM yourself but then you'll be lowering your own security and your proxy will have to do its own certificate validation because the browser won't be able to do it anymore. You are also restricted to things you can modify with a simple regex (unless you add HTML parsing to your proxy but then you'll be double parsing, once in the proxy and another time in the browser). And it's still probably going to break with websites in the HSTS preload list. And content generated by JavaScript won't be blocked easily. It's also going to be very inefficient, don't underestimate the years of performance improvements behind adblocking extensions. Adblockers like uBlock Origin also do much more than just blocking requests. For example they can inject small snippets of JavaScript to neutralize tracking scripts without breaking websites that depend on them by introducing dummy functions with the same API as the tracking script or to counter anti adblocking scripts. It can also inject CSS snippets to fix website breakage. And block requests based on what website they originate from. And probably much more than you can easily do with a simple proxy.