Gecko uses XPCOM extensively (though less than it used to). XPCOM is basically a cross-platform implementation of the same principles as Microsoft COM. In part

Gecko uses XPCOM extensively (though less than it used to). XPCOM is basically a cross-platform implementation of the same principles as Microsoft COM. In particular, XPCOM allows easy binding with the Spidermonkey JS runtime so you can expose things to scripts.