I had downloaded the Seamonkey 64-bit contributed build, and untarred/bzipped in my /usr/lib64 directory and then attempted to launch, only to see the dreaded:
Couldn't load XPCOM.
A search on the web lead to many suggestions of re-installing, which I considered to be using a machete when a scalpel should suffice.
So a little more digging suggested the problem is the ability of the program to link with the correct version of xulrunner. The seamonkey subdir does contain its own libxul.so and run-mozilla.sh, but apparently those are not interfacing with the rest of the system well.
So a peek inside /usr/lib64/xulrunner showed that it, too has a version of run-mozilla.sh. So I simply executed:
$ /usr/lib64/xulrunner/run-mozilla.sh /usr/lib64/seamonkey/seamonkey
and problem solved.
This, I'm sure, is an entirely unsophisticated description and understanding of what's going on, but the solution works (at least for me on my system).