Functions for getting the page HTML, search/replacing domain strings, and saving text to a file. Tested in Chrome 49 via Tampermonkey.

savePageToFile.js

window.addEventListener('load', function() {
    
    var data = getPageHtml().replaceUrls('foo.com', 'bar.com');
    var file = document.location.pathname.basename();
    //saveToFile(data, file);
    
}, false);


/**
 * Case-insensitive global string search/replace.
 * The search includes copies of the url prefixed with: 'http:', 'https:', '//', and 'www.'
 * @param {string} oldUrl - The url or domain string to be converted to regex and searched for. 
 * @param {string} newUrl - The string to replace any matches with.
 * @return {string} the 
 */
String.prototype.replaceUrls = function(oldUrl, newUrl) {
    var domainStr = oldUrl.replace(/^\s*(https?:)?(\/\/)?(www\.)?/i, ''); // Strip scheme/slashes/www
    var searchRegex = RegExp('(https?:)?(\/\/)?(www\.)?'+domainStr,'ig'); // Add optional scheme/slashes/www
    return this.toString().replace(searchRegex, newUrl);
}

/** Save the given data to specified filename. */
function saveToFile(data, filename) {
    var blob = new Blob([data], {type: "octet/stream"});
    var url = window.URL.createObjectURL(blob);
    
    var a = document.createElement("a");
    a.style = "display:none";
    a.href = url;
    a.download = filename ? filename : '';
    document.body.appendChild(a);
    a.click(); // Download!
    a.remove();
    
    window.URL.revokeObjectURL(url);
};


/** @return {string} the html tag and it's contents as an HTML string. */
function getPageHtml() {
    return document.documentElement.outerHTML;
}

/**  @return {string} the final path component */
String.prototype.basename = function() {
    return this.toString().replace(/\/$/, '').split('/').pop();
}