Downloading the Firefox browser now from the Mozilla website, users will find a unique identifier on the installer, which is submitted to Mozilla during installation and first run. The identifier, known internally by Mozilla as the dltoken, is used to associate downloads with the installation and first run of the Firefox browser. This identifier is unique to each Firefox browser installer, which means it is submitted to Mozilla whenever it is used.
While it is possible to download a new installer each time a new Firefox browser version is released, it is also possible to reuse the downloaded installer for this purpose. A bug report on Mozilla’s official bug tracking site confirmed the use of download tokens. The linked document is not public, but the listing itself confirms this use and provides an explanation as to why it was implemented:
This data will allow us to link telemetry IDs with download tokens and Google Analytics IDs. This will allow us to track which installs are produced by which downloads to determine answers to questions such as “Why do we see so many installs per day but not so many downloads per day?”
According to Mozilla, the identifier is used to analyze download and installation trends, among other things. This feature is provided by Telemetry in Firefox and is available on all Firefox channels. Interested users can verify these findings.
One of the easier ways is to check the hashes (same version, language, and architecture) of two or more Firefox browser installer downloads. Every hash value is different. Search for dltoken with any hex editor and you will find the string in the Firefox installer.