Partners include people like you and me that work with them on specific experiments. Depending on the kind of data, for example Normandy [1] data, Nimbus [2] data are used on those experiments and help to develop and improve the product. Results of those specifically are usually private [3], but other telemetry are public [4] which also collects what could be considered PII.
[1]: https://mozilla.github.io/normandy/
[2]: https://experimenter.info/faq/general-faq