There is no automated way to map your custom events or dimensions manually.
You need to manually map your customEvent:firebase_event_origin to a custom dimension when you configure the importer.
A event would probably be in the action scope, and dimension in a visit scope. Just be aware of the scope limitation:
Custom dimensions with scope=item is ignored during import as Matomo currently supports visit and action level dimensions only.
Limitations when importing google-analytics data
Depending on if you're using Matomo Cloud or On-prem, Matomo has 5 to 15 CustomDimensions per default, if you're expecting more than that, you would probably have to increase that number as well.