/sdcard/FxDown 

Here's what I've recommended in the past. Hopefully it still works in Firefox 38! Changing the download location on Firefox for Android requires a visit to the about:config preferences editor. You have to change two different settings to make this work. ''First, you need to know the folder path you want to use. An app like ES File Explorer File Manager will be helpful if you do not have a file manager.'' (1) Open a new tab to '''about:config''' (type about:config in the Awesomebar and tap the go arrow or tap Enter). ''You may want to bookmark this for easier future access.'' (2) In the search box, type '''down''' and pause while Firefox filters the list (3) Scroll down to '''browser.download.folderList''' and tap the preference. Up and down controls should appear. Tap up to change the value from 1 to 2. (3) Scroll back up and tap the large "+" button to the left of the search box to add a new setting (A) The name of the preference should be '''browser.download.dir''' (this stores the last used directory) (B) On the right side, tap Boolean and in the popup change it to String (C) The value depends on the folder you want. For example: /sdcard/FxDown (D) Then click Create (if you are too slow, it may clear and you might have to start again) If you switch back to your other tab and download something, it should go to the new folder. Any luck?