why Firefox runs this code 10x faster than Chrome

This quick n dirty code is already significantly faster in v8. (~24ms for 1000x1000 dataset)

var calc_histogram = function() {
    for(var x = 0; x < width|0; x++) {
        for(var y = 0; y < height|0; y++) {
            var  i = ((y * width + x) * 3)|0;
            var  r = data[i]|0;
            var  g = data[i + 1]|0;
            var  b = data[i + 2]|0;
            var  green = ((g > 80) && (g > (r + 35)|0) && (g > (b + 35)|0))|0;
            x_histogram[x] += green|0;
            y_histogram[y] += green|0;
        }
    }
};

|0 ensure that the number is an integer, it is asm js technique. Calling an array with a number require to make sure it is an integer, using |0 makes it explicit.

EDIT : And this is the fastest I manage to get without unnecessary |0. ~4ms for 500x500 and ~11 for 1000x1000. Note that I inverted the loops so it reads data in sequence to take advantage of prefetch, and I also used a bigger dataset to make improvements noticeable.

var calc_histogram = function() {
    var i=0;
    for(var y = 0; y < height; y++) {
      for(var x = 0; x < width; x++) {
            var r = (data[i|0]+35)|0;
            var g = data[(i+1)|0];
            var b = (data[(i+2)|0]+35)|0;

            if((g > 80) && (g > r) && (g > b)){
              x_histogram[x]++;
              y_histogram[y]++;
            }
            i=(i+3)|0;
        }
    }
}

I'm from 2021. I am currently using Version 87.0.4280.88 for Chrome and 84.0.2 for Firefox on an i3 Quad Core CPU 64 bit system.

I tried your code and the result is this for Chrome:

And this for FireFox:

So yeah, as of right now the speed results are pretty similar. But as a bonus I made some test code:

    console.time("speed test");
    for(let i = 0; i < 100000; i++) {
      console.log(i);
    }
    console.timeEnd("speed test");

The results are pretty surprising.

Chrome: 2527.755859375 ms

FireFox: 15687ms