FR: Support moz-extension as well as chrome-extension.

### [REQUIRED] Describe your environment

* Operating System version: Mac OS X 10.15
* Browser version: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:82.0) Gecko/20100101 Firefox/82.0
* Firebase SDK version: 7.24.0
* Firebase Product: auth (auth, database, storage, etc)

Firefox extensions use the scheme moz-extension but Firebase auth only works with chrome-extension.

Maybe there needs to be a web-extension URI scheme?

Anyway. Firebase auth needs to support this as our chrome extension works fine but won't work in Firebase.

This should be a super simple fix and I can just submit a PR.

#### Steps to reproduce:

#### Relevant Code:

https://stackblitz.com/fork/firebase-issue-sandbox

```javascript
// TODO(you): code here to reproduce the problem
```