Installing xmlextras on Thunderbird

I have a Gentoo Linux install with Firefox and Thunderbird. Firefox 
seems to come with the xmlextras component already installed, but 
Thunderbird doesn't for some reason.

How can I install this component manually so that Thunderbird recognizes it?

I tried installing it myself by copying the files xmlextras.xpt and
libxmlextras.so from my Firefox components directory (which does have
this installed) and running regxpcom.

Then I checked to make sure the components were installed in xpti.dat,
and they are now. But still I get the error XMLSerializer is not
defined from the Javascript console. What am I missing?

PS. Both Thunderbird and Firefox come with their own version of 
xpcshell, but I can't get either to recognize any xmlextras components, 
even though it does work in the Firefox Javascript window!