Altering HTTP Responses in Firefox Extension

Question

How can I alter the HTTP response body in a Firefox extension? I have setup an http-on-examine-response observer and an nsIStreamListener object with the code below. After I

Answer 1

For future readers looking for a way to do this in Firefox Quantum, there is an API that lets you filter responses. Using the method for long documents mentioned here, I was able to reliably change what I needed in my (temporary) plugin's background.js like so:

browser.webRequest.onBeforeRequest.addListener(
    function fixenator(details) {
        let filter = browser.webRequest.filterResponseData(details.requestId);
        let decoder = new TextDecoder("utf-8");
        let encoder = new TextEncoder();
        let str = '';

        filter.ondata = event => {
            str += decoder.decode(event.data, {stream: true});
        };

        filter.onstop = event => {
            str = str.replace(/searchPattern/g, 'replace pattern');
            filter.write(encoder.encode(str));
            filter.close();
        }

        return {};
    },
    {
        urls: ['https://example.com/path/to/url']
        //, types: ['main_frame', 'script', 'sub_frame', 'xmlhttprequest', 'other'] // optional
    }
    , ['blocking']
);