Martin Brinkmann, writing for gHacks:
Internet users who download the Firefox web browser from the official Mozilla website get a unique identifier attached to the installer that is submitted to Mozilla on install and first run.
The identifier, called dltoken by Mozilla internally, is used to link downloads to installations and first runs of the Firefox browser. The identifier is unique to each Firefox installer, which means that it is submitted to Mozilla whenever it is used.
Cool. I guess this is just another reason to use something else.
Update 3/26/22: Michael Tsai points out that this is unlikely to be the case with Firefox for macOS because it would be difficult to accomplish with notarization.