update cesiumVersions.php / cesiumDownload.php

• .deb download updated to 1.6.7. 1.6.5 and 1.6.6 were buggy : https://forum.duniter.org/t/cesium-desktop-lancement-impossible-sur-certaines-distributions-linux-cesium-1-6-2/7233/14

• Firefox extension download points to Mozilla repository.

cesiumDownloads.php

@@ -19,7 +19,7 @@ $cesiumDownloads =
 	],
 	'firefox' => [
 		'title' => _('Firefox'),
-		'url' => sprintf('https://github.com/duniter/cesium/releases/download/v%s/cesium-v%s-extension-firefox.xpi', $cesiumVersions['firefox'], $cesiumVersions['firefox']),
+		'url' => 'https://addons.mozilla.org/firefox/addon/cesium/',
 		'img' => 'firefox2019.png',
 		'desc' => _('Module complémentaire pour Firefox'),
 		'extra' => ''