If you have performance issues; you might want to check whether you need to blame the browser or some of your extensions.
This is a known issue, and has been around since at least v57. There are multiple Bugzilla issues on it. The cause is known. It isn't extensions. The fix is just invasive, and apparently ongoing.
The thread-parent absolutely cast a wider net than warranted, but everyone in this discussion who has experienced this problem knows exactly which one we're talking about, and the rest are all, "I've never seen a problem!" or "It's probably just extensions." Meanwhile, there's reliably a sub-thread somewhere in the discussion on nearly every article about Firefox, about this problem.
I find it profoundly ironic that you comment down-thread that "humans are very prone to confusing things which affect them personally with the general case" about a thing which you haven't personally experienced. You talk as if those of us for whom this is a 100% reproducible problem are an edge case, based AFAICT solely on your own not suffering it, coupled with your (not incorrect) beliefs about people poorly attributing performance problems in general.
That juxtaposition is really galling.