The Mozilla Suite (codenamed "SeaMonkey") was discontinued. A new project, outside of (but not on bad terms with) the Mozilla Foundation, was started to continue development under the name "SeaMonkey" (now as a brand, not just as a codename). As far as I know, they use recent upstream versions of Gecko and thus automatically support HTML5 and so on.
This is only confusing to people who followed Mozilla development closely enough to have seen "SeaMonkey" used to refer to the Suite. I'd guess that it was somewhat inspired by the origins of the Mozilla project - "Mozilla" was the old codename for Netscape Navigator.