It looks like Gentoo policy to have ebuilds for Thunderbird extensions (consider Lightning and Enigmail). It would be nice if the Webmail extension got an ebuild.
Steps to Reproduce:
Both enigmail and lightning extensions need building, however webmail does not. The available xpi's for enigmail and lightning are for mozilla's provided thunderbird binary, which only runs on x86 and amd64 with emulation. That means non-x86 users couldn't have enigmail or lightning. Webmail works in all the architectures, so no need for an ebuild.
Oh, didn't realize that was the issue. Sorry for the bother then, and yes the downloaded XPI does work fine for me.