How are they getting our data?

By setting up small pieces of code that trigger when you use a given feature, and send a network request to Mozilla’s servers with either a single flag set to just show a feature was used, in general, or more additional data with context (e.g. how long the text is that users are putting into their new AI sidebar feature)

What is the nature of the data?

This section of their Privacy Notice explains what categories of telemetry data they collect.

Can we do anything in about:config?

None needed. The normal settings menu has you covered. Go to Settings > Privacy & Security > Firefox Data Collection and Use > Allow Firefox to send technical and interaction data to Mozilla