Yes, the intention is that thunderbird-x11 be compiled 32-bit; that's the purpose of the line "supported_archs i386 ppc" in the portfile. The surprising part, and the reason the build fails, is that when trying to create "now" from "now.o", it's forgetting to use the -arch flags that were used in every previous step, thus it's trying to make "now" 64-bit, which of course won't work because "now.o" is 32-bit as it should be.
Unfortunately this port has no maintainer so someone will have to volunteer to figure out what's wrong and submit a patch.
