Rename pref branch to more human-readable:
extensions.closedownloadtabs. -> extensions.closeDownloadTabs.
Be careful: no auto-migration, this is <del>Sparta</del> alpha-versions!

Script for manually migration (you can run it from Error Console):

Components.utils.import("resource://gre/modules/Services.jsm");
var prefs = Services.prefs;
var oldBranch = "extensions.closedownloadtabs.";
var newBranch = "extensions.closeDownloadTabs.";
prefs.getBranch(oldBranch)
	.getChildList("", {})
	.forEach(function(pName) {
		var oldName = oldBranch + pName;
		var newName = newBranch + pName;
		if(prefs.getPrefType(newName) != prefs.PREF_INVALID)
			Application.prefs.setValue(newName, Application.prefs.getValue(oldName, 0));
		prefs.clearUserPref(oldName);
	}, this);