That's not how GDPR works. This isn't gathering personal data, PII or anything similar so wouldn't fall under the scope of GDPR.

IP address can be PII.

What are they hooking the download tracking code to if not IPs?

If you sign up for a Mozilla account, providing PII, are you saying they then throw away the link between the install and original download?

They don't need to hook an IP to a tracking code for this sort of testing?

From what they've said it's very basic information they're looking to collect. "This installer was downloaded on X date. The installer was then used and run on X date." sort of thing.

No IP is needed to check anything regarding that.

Attaching an IP in the process somewhere for tracking would only be needed if you were checking things on a deeper level. For example, where a user might download the installer but is downloading and sending it to someone else, where it gets installed on a different IP/system.