I have some sympathy for those complaints: I've been using Firefox since 2004, but for whatever reason I can't use it on my lab computer. It's a ThinkPad t430 running windows 10, and on that machine Firefox will slow the machine to an absolute crawl even with only 3 tabs open. I spent an hour trying to diagnose it to no avail (I can't just install Linux, I have some serial hardware that is windows-only) before just giving up and switching to chrome.

However, this is the only time I've encountered this issue, and I tend to think it's more likely an issue with the windows installation than Firefox. I'm just too lazy to reinstall, this isn't my main computer after all.