To help improve the experience for new users, Mozilla uses a download token (a unique identifier) that is associated when downloading Firefox. This allows Mozilla to capture that a download has happened, which in turn helps to understand how many installs come through third-party sites and what those sites are. If you look in "Campaign and Referral Data" in the Firefox Privacy Notice under "Measure and support our marketing". This information enables Mozilla to determine the website domain or advertising campaign (if any) that referred you to the download page.
Mozilla does not keep the download token with any personally identifying information, and the download token is stored in a database that is separate from other telemetry information under a strict access policy.
If you are interested in removing your association with the download token you can opt out of Firefox telemetry once you have installed the browser. Mozilla deletes all of the historical telemetry data that is collected for your Firefox profile, including all records containing the download token. This makes it impossible for Mozilla to associate the web site visit with that profile, even for activity that took place before the opt out was specified. Alternatively, if you want to prevent a download token association from ever happening in the first place, you can download Firefox from our archive site.