Goodbye Google Analytics?

You can use firebase with an ANE. There is no way to use the Firebase Analytics without a Native library as there are a lot of data for mobile, which weren't available in Analytics.

Implementation is as easy as it gets. Just add the Firebase Core ANE, FirebaseAnalytics ANE and you are ready to go.

There are a few paid options and there is even a free one.

https://forums.adobe.com/people/Douglas+McCarroll  wrote Hi All, I just went to my Google Analytics account to create a new "mobile property" and discovered that GA is being deprecated for mobile apps and I'm now supposed to use Firebase.

the type of the property, in the past, mainly changed some default views you have in GA
and then GA decided to share the data whatever the property

google+ post: Understanding User Behavior in a Multi-Device World

Analyze app and web data in the same reporting view Any data you send to the same property appears in all of the reporting views, regardless of how you collected that data. This means that if you send data from the web or from a mobile app to one property, both data sets appear in your reports. If you want to isolate data from one source, like if you only want to see web data in your reports, you can set up a filter to customize what you see. You can also use other tools to isolate each data set, including customizations in standard reports, dashboards, custom reports, and secondary dimensions.

you can use a website property to track mobile app, desktop app, even command-line and server app

as long as you send correct tracking data it will be reported

1.. Am I correct that it's no longer possible to create a new Google Analytics mobile property?

No, see it more like the website property being the general category or default
and the new mobile app property is for dev already using firebase so in integrates better

if you don't want to use firebase, just use a website property and send "typical" mobile app tracking

like screens, app name, app version, etc.

2. If I want to use Firebase for (just) analytics do I have to use an ANE?

probably, but technically it could be implemented all in AS3
it just nobody did that kind of work and in itself it is probably "safer" to use an ANE based on the Firebase SDK
to benefit from upgrades etc.

3. Is there any way to us ActionScript code such as Zwetan's GA code (https://github.com/zwetan/as3-universal-analytics) with Firebase?

Not the same thing, not the same API

in the doc Measurement options for mobile apps
see the part

Use the Measurement Protocol The Google Analytics Measurement Protocol is a standard set of rules for collecting and sending hits from any Internet-connected device to Analytics. It's particularly useful when you want to send data to Analytics from a kiosk, a point of sale system, or anything that is not a website or mobile app. Because, while the Analytics JavaScript and mobile SDKs automatically build hits to send data to Analytics from websites and mobile apps, you must manually build data collection hits for other kinds of devices. The Measurement Protocol defines how to construct the hits and how to send them to Analytics.

as3-universal-analytics implements and use that Google Analytics Measurement Protocol,
the deprecated Google Analytics Services SDK also used it, the Firebase SDK certainly use it too

the difference is the Google SDKs automatically build hits for you

and by using a library in AS3 you will have to build those hits more manually

but underneath everyone use the same collection protocol

see this page with more details about the differences
GoogleAnalyticsServicesSDK · zwetan/as3-universal-analytics Wiki · GitHub

4. Are there any other analytics options out there that you would recommend?

many more options exist, depends on what you need
GA has already lot of functionalities and features so personally I did not explore/compare all other solutions

with GA and a lib like as3-universal-analytics you will have more setup to do,

like custom dashboards in GA, manually write AS3 code for the tracking,

but then you will have full control of what/when/how it is tracked, you will know the details

and it will just work anywhere the library can work

as long as Google does not change/update/deprecate the measurement protocol
which is unlikely to happen

the Google SDKs will always force you to use an ANE, update/upgrade it,

change it if the SDK get deprecated, etc.

so pros and cons, I prefer the pros of using an AS3 lib