Firebase : Read-only & non-realtime mode activated to improve browser performance
If you want to delete a high level node when this is activated, I recommend doing this.
Open up a text editor and type in { }. Save this file as "blankJSON.json".
Go to high level node you want deleted and select it, once it opens up and shows you all the nodes that need to be removed, select the three bars at the top right and select "Import JSON", (It would be safe to first "Export JSON" If you don't have backups, in case you make a mistake here). Import the JSON file we created earlier titled "blankJSON".
This will delete all of the data inside.
Once again, I highly suggest you make a backup before doing this, It's extremely easy to make a backup and also it is much easier than you would think to upload this blankJSON to the wrong node and then erasing a bunch of important data.
When it detects that it's downloading too many nodes from your database, the Firebase Console stops using real-time mode and switches to read-only mode. In this mode it requires less work from the browser, so it is more likely that the browser will stay performant.
To get back to realtime mode, you have to go to a location that has fewer nodes. So say that you start loading the database at the root, that means that "pseudo address bar" at the top of the data tree will say:
https://<your-project>.firebaseio.com/
And then will show the list of items. Now click on the URL in that pseudo address bar and change it to:
https://<your-project>.firebaseio.com/<one-of-your-keys>
And hit enter. The data tree will reload with just the node from one-of-your-keys and down and will likely switch to realtime mode again.
Every node key in firebase is a link, you can open a sub-node in a new tab and then edit that sub-node and its children.
Right click on a sub-node you want to edit or delete
Select open link in a new tab
Edit the sub-node in the new tab