Firefox for Android supports setting a custom folder via about:config. Set browser.download.folderList to 2 and browser.download.dir to a valid linux path. For example /storage/emulated/0/test works on my phone, you will need to determine the correct path for your device.

Take care that the preferences and paths do not end up with extra spaces, some keyboards will attempt to insert spaces after the . characters or if autocomplete/autocorrect happens.