“Firefox 120 was the breaking point since Firefox 119 and prior ended up running into issues with Selenium / Gecko web driver for automating the benchmarks.”
Phoronix benchmarked every major Firefox release from 120 through 141 beta on an AMD Ryzen 9 9950X, covering nearly two years of development. The testing was prompted by Firefox 141 showing lower RAM usage on Linux. This is the kind of methodical performance tracking that browser vendors should be doing publicly themselves. Instead we get marketing blog posts about how every release is faster than the last, and it takes an independent tester to show what actually changed.