I'm afraid I can't speak for HockeyApp at all, but I have some experience in using Firebase and Crashlytics using iOS and Android clients. The below paragraphs don't factor in HockeyApp, and only compare Firebase Crash Reporting to Crashlytics.
Of the two, I would depend on Crashlytics for crash reporting until Firebase can further revise features. I've implemented both in some apps, and there are some advantages to Crashlytics. They send email notifications when crashes occur, including for priority changes, and crashes appear very quickly in the interface, within 5 minutes typically.
As it currently stands, Firebase doesn't have crash notifications, and it can take 20-40 minutes for a crash report to appear in the dashboard. A benefit of using Firebase's reporting is that their analytics will create an audience group of crash experiencing users, allowing you to identify and provide a different experience for those users (push notification, welcome screen, coupon code, etc)
Both:
Ability to report logs for crashes to investigate interaction and
function
Crashlytics:
~5 minute reporting time
Email notifications
Firebase:
20-40 minute reporting time
Richer user data
Can redact some logged info: "Logged in Chris" could become "Logged in [REDACTED_US_MALE_NAME]"