How did you download it? Does it appear in Firefox's download manager (Ctrl+j)? In that case you should be able to find it by right-clicking it and choosing Open Containing Folder. Your browser is identifying your OS as Windows NT 4, which is very rare these days. If it is incorrect (e.g., you run Windows 2000, XP, Vista or 7), then this article might help: [https://support.mozilla.com/en-US/kb/Websites%20or%20add-ons%20incorrectly%20report%20incompatible%20browser Websites or add-ons incorrectly report incompatible browser | Troubleshooting | Firefox Help].