SeaMonkey seems to load the messenger fine so you guys have decent seamonkey internet suit support (which is true considering SeaMonkey is the community supported continuation of Mozilla Internet Suit which is what the Firefox browser is based on)
and Netscape support is no problem because Mozilla has its roots in Netscape and the current Netscape is in turn based off Firefox
so you should be able to render xul in SeaMonkey/Mozilla and Netscape just fine though I haven’t tested Netscape so I don’t know (the most current Netscape on this site anyway)