* Java properties files can be translated in OmegaT (but I'm not sure how Mozilla properties files would fare).
* The Mozilla properties and DTD files can be converted to PO and back using moz2po from the Translate-toolkit. Those PO files can then be translated in, say, Pootle... which is quite a job to install but fairly easy for managing translation projects (perhaps pootle.babelzilla.org is a possibility?). I use Pootle on MS Windows XP, running Pootle as root. Pootle's UI is the user's browser.