Browser.chrome.image icons.max size
From MozillaZine Knowledge Base
To help users differentiate between images loaded in tabs, Firefox sets the tab icon (and the icon in the Location Bar) to a small version of the image. Early tests of this feature revealed performance problems; to address this, Firefox only shows the image’s thumbnail in the tab and Location Bar if the image is smaller than a certain size. This preference determines the cutoff.
This preference may become obsolete in the future if bug 305986 is fixed. (This bug is fixed in SeaMonkey since 2008-09-25. The fix may or may not have been ported to Firefox.)
Possible values and their effects
This preference takes a positive integer. If an image’s width or height is greater than this number, the default icon is displayed instead of a thumbnail. The default value is 1024. Setting it to 0 will disable image thumbnails.
The default value should be a good compromise between performance and usability. If you experience slowness when viewing images less than 1024 pixels high or wide, however, you may want to decrease the value of this preference (or set it to 0).
First checked in
Has an effect in