If you still need libgtkmozembed for any other application, you might reconsider to use Seamonkey 1.1.18 rather than upgrading to Seamonkey 2.0. I don't use Seamonkey on daily basis, so i have no problem using 1.1.18 instead of having problem with other applications due to removal of libgtkmozembed.
Here's the latest -Current changelog:
Wed Nov 11 21:17:19 UTC 2009
As of version 2.0, SeaMonkey no longer includes a shared libgtkmozembed library, so the pkgconfig and header files for this have been removed from the package to avoid confusion over why trying to compile programs that use it no longer works. There's not really any way to avoid this issue --it's either upgrade SeaMonkey and lose libgtkmozembed, or keep using version 1.1.18 and miss out on SeaMonkey security updates. If you really need libgtkmozembed, the seamonkey-1.1.18 packages will be retained in either /patches or /pasture for Slackware versions 13.0 and earlier.