Connect Firebase Hosting to Namecheap Custom Domain

SOURCE, SOURCE, SOURCE

1. Go to Firebase's Dashboard > Develop > Hosting > Connect Domain.
2. Enter the custom domain that you'd like to connect to your Hosting site.
3. (Optional) Check the box to redirect of all requests on the custom domain to a second specified domain.
4. Click Continue to initiate the validation process.
5. Go to Namecheap's Dashboard > Domain List > Manage > Advanced DNS.
6. Under Host Records, add a new record with:

   type: TXT Record
   host: @
   value: copy from Firebase's dashboard
   TTL: 60 min
   

7. Wait for 10 - 20 mins, go back to Firebase's Dashboard click Verify.
8. Copy the 2 provided A records into Namecheap's Dashboard with the following value:

   type: A Record
   host: @
   value: copy from Firebase's dashboard
   TTL: Automatic
   

9. Remove the default CNAME Record for parkingpage.namecheap.com and the default Redirect record.
10. Repeat the same setup from step 2 for www.yourdomain.com but instead choose redirect to your previously setup yourdomain.com.
11. Copy the 2 provided A records into Namecheap's Dashboard with the following value:

    type: A Record
    host: www
    value: copy from Firebase's dashboard
    TTL: Automatic
    

12. Create a CNAME Record with the following value (value is your main Firebase address with a dot at the end):

    type: CNAME Record
    host: www
    value: linene-xxxxx.firebaseapp.com.
    TTL: Automatic
    

Now you just have to wait for your hosting to update the DNS records and for DNS caches to flush. The status in your Firebase console should be Pending.

Also, keep in mind that your SSL certificate is automatically provisioned but requires a bit of time to be valid.

See THIS for example: