Firestore is a quickly changing offering but here is a simple way to grab some data from a Collection/Document structure in Cloud Firestore: tap Query Builder, use the dropdowns to select the scope and path and up pops a grid of data...depending on your plan and the size of data you are looking at, you might have a limit ($$ or otherwise) but at this point you can just copy the data grid and paste it into something like Excel for further manipulation (if you want).

This really helped me quickly get to the place where I would look at the documents all at one time.