custom-openstreetmap-tile-server/js/background.js

const urls = [
  "*://*.tile.openstreetmap.org/*"
]

var tileserver_url = "";

function log(msg) {
  console.log("custom-openstreetmap-tile-server: " + msg)
}

function setTileserver(storage) {
  tileserver_url = storage.tileserver;
  log("set tileserver_url to " + tileserver_url);
}

function onError(error) {
  log(`Error: ${error}`);
}

function updateTileserverUrl() {
  var getting = browser.storage.local.get("tileserver");
  getting.then(setTileserver, onError);
}

function replaceTileserver(request) {

  updateTileserverUrl();

  if (tileserver_url == "") {
    log(request.url + " -> no tileserver_url");
    return {}
  }

  var url = request.url.replace("tile.openstreetmap.org", tileserver_url);
  log(request.url + " -> " + url);
  return {redirectUrl: url};
}

chrome.webRequest.onBeforeRequest.addListener(replaceTileserver, {urls: urls}, ["blocking"]);