They've said many times they don't log IP addresses so if they do, it's definitely illegal. So let's look at the risk vs reward of that situation:

Risk: very high because they rely on user trust to survive and they could get sued into oblivion under the GDPR.

Reward: they now have a list of IP addresses with Firefox users behind them. Any big website has that so it's not valuable to third parties and it's not valuable to Mozilla because they don't do any targeted advertising or anything similar.

A list of IP addresses is about the least useful thing to log if you want to track people.