userChrome.css still works, and if you already had one they automatically flip the pref on for you.

It's default-off because it increases startup time.

I wonder, what's the 95% worst case latency for a stat on a 5400 RPM laptop drive from 10 years ago? Assuming of course that the drive is already spinning since it's reading a million other things from your profile.

I bet it's negligible in almost every case.

With a 5,400 rpm, just the slow rotation can introduce a wait time of 11 ms. Seek time from moving the arm can incur up to 20 ms (although some of that might overlap with the rotation time). And if it's a ten year old HDD, there might be a re-read necessary.

Even if the average read time would be just 20ms per file, cutting out ten of these reads saves 0.2 seconds, which is already noticeable to humans. And that's under the unrealistic assumption that Firefox is the only program trying to read from that poor slow HDD.

On a sidenote, laptops with 5,400 RPM drives are still sold.

> On a sidenote, laptops with 5,400 RPM drives are still sold.

And in the case of Apple also 4k iMacs. They even knowingly hide it now via a (non-existing?) footnote, seemingly hoping nobody clicks the "technical specs" button.

A single file open and close isn't that much, but dozens or hundreds of these tiny performance papercuts add up and make application startup slow.