Except that Adblock Plus wasn't the problem when Firefox was slow with web apps or sites that used JavaScript to load content. Firefox's JavaScript engine lagged significantly behind Chrome (and Safari) for a few years. It still does in a few areas.

That's partially true but remember that you're generalizing from micro-benchmarks – which did not universally tilt Chrome – to the complex behavior of real sites, where networking, memory usage, and general de-optimization tended to cancel out many of those small wins.

That's not saying that the browser teams haven't made big strides, only that it's not such a simple story. Chrome running a certain JavaScript function 15% faster isn't a game changer if a side is limited by layout or poorly structured DOM interactions.

Aren't micro-benchmarks kind of pointless if they don't reflect real-world behavior?

The fact is that javascript execution (not networking behavior but javascript performance) on many web apps differs by orders of magnitude between Firefox and Chrome. There are a handful of sites that I find literally unusable in Firefox on a new Macbook Pro because even after loading, sitting in the background doing nothing, they make the entire computer unresponsive due to high CPU usage. These same sites work just fine in Chrome and Safari.

Well, for years, uninstalling AdBlock Plus was unacceptable (at the time, now we have uBlock Origin). For many people, a browser without an ad blocker is an unusable browser.

There were other options which had much better performance, but that's not really the point: much of what people blamed on the browser was actually a less visible program they'd installed, similar to the way Windows often got blamed for performance or stability problems created by anti-virus software.