/sdcard/Download

Unfortunately, you may need to rename the file with an appropriate extension. For example, it might be .pdf or .doc or .xlsx depending on what application it is designed for. To do that, you can use a File Manager app. For example, I use ES File Explorer: https://play.google.com/store/apps/details?id=com.estrongs.android.pop Firefox uses the same Downloads folder as other programs on your system. Not sure about other Android versions, but mine's at /sdcard/Download If you long press a file in ES File Explorer, a toolbar will appear at the bottom which allows you to rename the file. Unfortunately, the entries in Firefox's download list and the Android Downloads app will no longer work after that, you'll need to access the file through the file manager.