FR: Add Support for Firefox Extensions (moz-extension://)

### [REQUIRED] Describe your environment

* Operating System version: MacOS 10.14.5
* Browser version: Firefox 70.0.1
* Firebase SDK version: 5.11.1
* Firebase Product: Auth

### [REQUIRED] Describe the problem

When try you to use Auth inside a Firefox Extension, you get the following error:

```This operation is not supported in the environment this application is running on. \"location.protocol\" must be http, https or chrome-extension and web storage must be enabled.```

This origin check should also allow `moz-extension`, which is identical to `chrome-extension`, just inside Firefox. Happy to create a PR if it will be accepted.