Firefox implements the filter effects working draft <feDropShadow>
element. This is optimised so it will work somewhat faster than tying all the individual filters together.
The example below shows the an SVG 1.1 drop shadow and the new <feDropShadow>
equivalent.
<svg viewBox="0 0 800 400"
xmlns="http://www.w3.org/2000/svg">
<defs>
<filter id="feDropShadowEquiv1">
<feGaussianBlur stdDeviation="3"/>
<feOffset dx="3" dy="3" result="offsetblur"/>
<feFlood flood-color="#720"/>
<feComposite in2="offsetblur" operator="in"/>
<feMerge>
<feMergeNode/>
<feMergeNode in="SourceGraphic"/>
</feMerge>
</filter>
<filter id="feDropShadow1">
<feDropShadow stdDeviation="3" flood-color="#720"/>
</filter>
</defs>
<g font-family="Verdana" font-size="30">
<text x="100" y="90" filter="url(#feDropShadow1)">feDropShadow</text>
<text x="100" y="140" filter="url(#feDropShadowEquiv1)" >feDropShadow</text>
</g>
</svg>