They had to serve you the file, for which they needed your IP. If they're willing to assign each downloaded client a unique ID what are the odds they are not storing the IP address associated with that unique ID?
Easiest explanation off the top of my head, without reading the article, would be IT departments including Firefox in their base image they use on all their standard issue computers, resulting in hundreds and possibly thousands of different installs having the same download ID. That alone by itself would cause an absolutely massive discrepancy between download and install numbers. My company includes Firefox in our base image and it's on at least 200,000 different laptops and desktops, with a handful of different download IDs between them depending on when they got issued the computer.
Also, they could just ask, instead of adding more spyware. Like a survey.
(Joking, but only a little.)