cordova-plugin-firebasex provides support for push notifications, analytics, event tracking, crash reporting and more from Google Firebase. This sounds great in theory, however including all of this functionality leads to very long compiling times, especially with iOS. With Appflow the use of this plugin will commonly result in build failures due to the 60 minute time limit being reached for iOS builds.
It is worth noting that this plugin does include a workaround for this long compiling time issue by using the variable IOS_USE_PRECOMPILED_FIRESTORE_POD in the config.xml file. The issue here however is that cordova-plugin-firebasex requires using Cordova hooks which is not supported by Appflow.
Because of this incompatibility, the recommended course of action is to look for alternative plugins that provide only the functionality you need rather than all the functionality provided in this monolithic plugin. This solution also lines up with the general best practice of only including plugins that provide the functionality needed for your project which helps improve maintainability. At the time of this writing, this was a good resource for alternative plugins for firebase.