Replace text in a website

You could perform your repleacements on all the just the text nodes in the DOM:

function replaceTextOnPage(from, to){
  getAllTextNodes().forEach(function(node){
    node.nodeValue = node.nodeValue.replace(new RegExp(quote(from), 'g'), to);
  });

  function getAllTextNodes(){
    var result = [];

    (function scanSubTree(node){
      if(node.childNodes.length) 
        for(var i = 0; i < node.childNodes.length; i++) 
          scanSubTree(node.childNodes[i]);
      else if(node.nodeType == Node.TEXT_NODE) 
        result.push(node);
    })(document);

    return result;
  }

  function quote(str){
    return (str+'').replace(/([.?*+^$[\]\\(){}|-])/g, "\\$1");
  }
}

Quote function borrowed from this answer.

Usage:

replaceTextOnPage('hello', 'hi');

Note that you will need to SHIM forEach in older browsers or replace that code with a loop like so:

var nodes = getAllTextNodes();
for(var i = 0; i < nodes.length; i++){
    nodes[i].nodeValue = nodes[i].nodeValue.replace(new RegExp(quote(from), 'g'), to);
}