Firefox doesn't show broken images by default, it shows only images' alt tag as simple text. This userstyle can help to show broken images with their height, width and alt, just like IE, Opera, Safari. alt tag has fixed font.
Firefox默认不显示链接失效的图片，只显示失效图片的alt标签为普通文本。此userstyle可使之显示链接失效图片，如果有设定高、宽、alt也将其显示，就像IE、Opera、Safari一样。显示的alt标签字体固定。

You can also use the “Image fix and linkify” Greasemonkey script to linkify broken images:
你也可以使用这个“Image fix and linkify”Greasemonkey脚本，将链接失效图片变成链接：
http://userscripts.org/scripts/show/23775