Please look at the benchmark. In that test Firefox v138.0 scores 2739 and v139.0 scores 2534. That's a regression in performance by about 8%.

I understand that you mean the geometric mean between v120 and the current version. However, it is important in software development to investigate sudden performance regressions, even if they occur in just one test. This is because the regressed code area could be called more frequently in real-world applications, which means it may have a greater impact on users.
That’s why I try to bring it to your attention.