An error message about XPCOM missing or about platform version usually happens because of a failed Firefox update that left you with a mixture of old and updated files in the Firefox program folder. This can happen in case security software was keeping a handle of some of the files in the Firefox program folder preventing Firefox from replacing all the files. You can do a clean reinstall to ensure that all files are replaced. You may have to disable security software temporarily in case the problem persists. You can find the full version of the current Firefox release (56.0) in all languages and all operating systems here: *https://www.mozilla.org/en-US/firefox/all/ *https://www.mozilla.org/en-US/firefox/notes/