How can I upload a file to a GitHub Codespaces environment?

Simple Drag and Drop has worked for me.

To update the answer, there is now also an update option if you right click in the explorer view:

I was able to transfer a ton of training data from firebase to a codespace using firebase-admin.

in the codespace terminal:

pip install firebase-admin 
pip install --ignore-installed <other firebase-admin dependencies>
run whatever script you use to download things from firebase 

note that there are storage limits