The key is to dimish pictures quality via custon css-file.

1. Open about:support and there click and open profile folder.

2. Make a chrome folder inside profile folder

3. Make a userCrome.css file which contents is

img {
/*image-rendering: auto;*/
/*image-rendering: crisp-edges;*/
image-rendering: pixelated;
/* Global values */
image-rendering: inherit;
/* -webkit-filter: grayscale(1); */
/* -moz-filter: grayscale(1);*/
/* -ms-filter: grayscale(1);*/
/* -o-filter: grayscale(1);*/
/* filter: grayscale(1);*/
/* filter: url(grayscale.svg); /* Firefox 4+ */
/* filter: gray; /* IE 6-9 */
}

….save and quit

4. Open about:config on there set

toolkit.legacyUserProfileCustomizations.stylesheets

as true.

5. Restart Firefox

If you want Firefox even faster so that it does nod load pictures at all, open about:config and set

permissions.default.image

as 2

The other options are

1 — Always load the images

2 — Never load the images

3 — Don’t load third images

https://support.mozilla.org/en-US/questions/981640

Also disabling web rendering can make Firefox and Seamonkey faster, but it can cause blurry fonts which makes sites look like in Windows.

about:config change

gfx.text.disable-aa

as true

Picture from Seamonkey

Rasterization disabled as true and raster false