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.
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.