Firefox xlink:href bug???

Hello guys! I'm getting somethings done with SVG in Firefox. But,
there is something I can't get it work. Please help me, thank you. The
problem here is that: it seems that Firefox doesn't understand <use
xlink:href="OuterFile.svg" x="xxx" y="xxx" />. I use the <use> tag
inside a svg file and xhtml file. But they don't work at all, nothing
is shown (I want to show another SVG file in another SVG file, or in
another XHTML file). I've tried many times, and I'm sure that I
included enough name space in <SVG> tag.
I really don't understand why it didn't work. Your help would be very
appreciated. Thanks again!