Returning a Promise of a BlockingRequest object does not work if the Promise object is provided by babel-runtime (Firefox 52.0.2). When using background-source.js instead of background.js in manifest.json redirection of example.org does work fine.

background-source.js

function failingListener() {
    return Promise.resolve({
        redirectUrl: 'http://mozilla.org'
    })
}

function functioningListener() {
    return {
        redirectUrl: 'http://mozilla.org'
    }
}

browser.webRequest.onBeforeRequest.addListener(
    functioningListener,
    {urls: ['*://*.example.com/*']},
    ['blocking']
)

browser.webRequest.onBeforeRequest.addListener(
    failingListener,
    {urls: ['*://*.example.org/*']},
    ['blocking']
)

manifest.json

{
    "name": "PromiseRedirectTest",
    "version": "1.0",
    "background": {
        "scripts": ["background.js"]
    },
    "permissions": [
        "<all_urls>",
        "webRequest",
        "webRequestBlocking"
    ],
    "manifest_version": 2
}

package.json

{
  "name": "PromiseRedirectTest",
  "version": "1.0.0",
  "description": "",
  "scripts": {
    "build": "browserify -o background.js background-source.js"
  },
  "devDependencies": {
    "babel-plugin-transform-regenerator": "^6.24.1",
    "babel-plugin-transform-runtime": "^6.23.0",
    "babel-preset-es2015": "^6.24.1",
    "babelify": "^7.3.0",
    "browserify": "^14.3.0"
  },
  "dependencies": {
    "babel-runtime": "^6.23.0"
  },
  "browserify": {
    "transform": [
      [
        "babelify",
        {
          "presets": [
            "es2015"
          ],
          "plugins": ["transform-regenerator", "transform-runtime"]
        }
      ]
    ]
  }
}