kernel2korn, I just did as nanotube suggested and downloaded the source from ftp.mozilla.org and compiled it from source using the Simple SeaMonkey build instructions. My .mozconfig file is as follows; you may need to adjust the value of the -j flag to suit your number of processors:
I used /usr/local/lib64 because that's where openSUSE puts its 64-bit binaries; I'm not sure what the convention is on Ubuntu. Anyway, then I just compiled and installed it as follows:
SeaMonkey installed to /usr/local/lib64/seamonkey-2.4 with a link to the binary from /usr/local/bin, and when I run that I still have all my plugins and extensions. I didn't uninstall the old SeaMonkey from the Ubuntu repositories, so maybe that has something to do with it.
$ make -f client.mk
$ sudo make -f client.mk install