Earlier quoted context omitted.
I swear this conversation is as old as time --- or at least Firefox & Chrome. person 1: "Firefox is slow now i'm moving back to Chrome" person 2: "interesting! I find the complete opposite and am moving back to Firefox!" :)
It often seems like this is because has dozens of old, forgotten extensions running in the background. They suck up memory and generally slow things down. Switching to erases this problem! For a few years, anyway... And then the cycle repeats.
Personally, I think the browsers actually do beat each other in performance every couple of months. i.e. Chrome Version 49.0.2623.87 (64-bit) is better than (current firefox version) but next month it'll be the reverse.