Perhaps this was due to their red-teaming partnership [1][2] with Anthropic which they wrote about a few months earlier in March?
1: https://www.anthropic.com/news/mozilla-firefox-security
2: https://blog.mozilla.org/en/firefox/hardening-firefox-anthro...
Previous discussion: https://news.ycombinator.com/item?id=47273854
I just did a search and apparently this fact (the specific one about no payouts for the first time in almost 20 years) has not gotten a discussion on HN. Given the degree of skepticism around the utility of AI bug finding and fixing (this very thread is full of it), I would have thought that concrete evidence that it can help actually make real software more secure against attacks would have gotten a write-up somewhere.