There's a long time during which Firefox was somewhat slow, and I remember the spidermokey team releasinf the famous Are we fast yet website that was saying no during this period.
I was so happy when Chrome came out. Over the years I've tried going back to Firefox and I've gone back to either Chrome, Ungoogled Chromium or Brave.
On Linux though, and blocking ads.
At the time 3.0 came out I was still in University and every other student I spoke to had the same experience. People also experienced in my first place of work in 2009.
This problem was ultimately fixed with 4.0 when they did something different with how multi-tab worked.