Great question and glad you reached out about this!
We did recently deprecate issue numbers, so those won’t be viewable on the dashboard anymore. The decision to drop support for it was based on customer feedback, and because we don’t plan to carry over this particular feature to Firebase.
Definitely understand if that feature was important to you. There might be a couple of things you can do here. If you look at the URL when you click into the issue details on one of your issues, you should see ".../issues/xxxxxxxxxxxxxxxxxxxxxx/..." which is the "issue ID," a hexadecimal string which acts as another unique identifier for that particular issue (on a per-app basis).
Alternatively, you could send the URL to your co-worker to link to the Fabric issue.
Let me know if you have any further questions :)