Firebase GTM vs Legacy GTM

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The Difference Between Firebase GTM and Legacy GTM for Native Apps

Google has made many additions to native app analytics in the last several months. The addition of the Firebase platform to the analytics environment brings about many new changes. One of the major changes is the introduction to Firebase Google Tag Manager (GTM). Anybody involved with Google native app data should be aware of these updates—especially if they work on data collection. Let's read on to understand what some of the new additions and pain-points are. When we attempt to create a new GTM container we are given two choices: Firebase GTM or Legacy GTM.   Note: The same options are available for Android. Android was not shown for brevity.   Legacy GTM is the older version of GTM while Firebase GTM is the newer option. Some of these additions are below: First and foremost, Firebase GTM allows for data to be sent to Firebase Analytics (which was rebranded to Google Analytics for Firebase—we'll refer to this as Firebase Analytics to avoid confusion). Once the Firebase and GTM SDK have been installed, data is sent to Firebase Analytics automatically. Legacy GTM has no ability to send data to Firebase Analytics. Now let's look into the tag differences.  


Firebase GTM offers many more tag templates than Legacy GTM.  

Firebase Google Tag Manager Tags

Legacy Google Tag Manager Tags

Universal Analytics Universal Analytics
AdWords Conversion Tracking AdWords Conversion Tracking
AdWords Remarketing AdWords Remarketing
DoubleClick Floodlight Counter DoubleClick Floodlight Counter
DoubleClick Floodlight Sales DoubleClick Floodlight Sales
Custom Image Custom Image
Function Call Function Call
Firebase Analytics
Apsalar Tag
Kochava Event Tracking
Tune Mobile App Tracking
As expected, Firebase GTM can send data to Firebase Analytics, but we can see above that there are many other vendors that are supported. At it's core, we have the same tag features between Firebase GTM and Legacy GTM. Firebase GTM simply has more templates and will be supported by Google moving forward. Next we'll look at triggers.  



Firebase Google Tag Manager Triggers

Legacy Google Tag Manager Triggers

Custom Custom
Container Loaded
App Update
First Open
In-App Purchase
Notification Foreground
Notification Open
OS Update
Session Start
User Engagement
We see that Firebase GTM has many more options than Legacy GTM. The weakness of Legacy GTM is the lack of  automatic triggering events. Everything requires a manual dataLayer push with a specific triggering event. While this approach should still be used in Firebase GTM, we have many more options. We can trigger a tag when the app is opened for the first time, when a session starts, or when the user purchases an item automatically. One thing to keep in mind is that these are based off Firebase Analytics definitions. For example, when an in-app purchase is recognized by Firebase Analytics, a trigger event will be sent automatically. Regardless, this is a welcome addition that provides much more flexibility.  


The main differences between variables are below:  

Firebase Google Tag Manager Triggers

Legacy Google Tag Manager Triggers

Event Name Custom Event
Event Parameter DataLayer Variable
Firebase User Property DataLayer Variable
Lookup Table
Firebase GTM uses the Event Name variable to trigger custom events triggers. Custom Event variables fulfill the same purpose in Legacy GTM. DataLayer variables are replaced by Event Parameters and User Properties. Firebase has a specific method call for tracking events and setting user properties. The information passed along with the event or user property is captured as an Event Parameter and User Property variable, respectively. They take the place of dataLayer variables. Lastly, Firebase GTM has Lookup Tables. These variables behave identically to Web GTM. We can map specific input values of a variable to specific output values. This allows significantly more flexibility than Legacy GTM as we can now more readily manipulate the data inside GTM.