Hi @edvinas, welcome to the community!
You can paginate through large results numbers using a structured query and then use suitable data to limit the results you’re getting from Firestore: StructuredQuery | Firestore | Google Cloud
For example, fetch only users with an ID between 0 and 1000, then 1001 and 2000, then 2001 and 3000 etc. You’d only need to find a suitable field in your documents.
To save memory in n8n you can then build a parent workflow that only keeps track of these values and do the heavy lifting in a sub-workflow instead. This example post explains the basic idea, but instead of calendar events you’d loop through Firestore documents of course.