May and June were good months for Firefox’s Speedometer performance compared to Chrome. We’re closing in while Chrome seems fairly static. In this visualization, lower in the graph is better. From https://arewefastyet.com/win10/benchmarks/overview?numDays=60.
I think it is so that on all graphs “lower” down is better. But agree it’s kind of unusual layout. Looking at the graphs for Linux i wonder what happened in feb 11:th this year - where all values for Firefox got a lot worse.
Sometimes the hardware or software configurations of the machines running the tests changes, or a bug in the test harness itself is fixed, which can skew all of the results at once.