Hi Charis

I would strongly recomend against installing .apk files outside of official well establish app stores as there is a great risk of malware ending up on your device.

This does not sound like an issue with Firefox for Android, but have you set Android to allow outside .apk files?

Of course i think it's firefox problem because i can download and install with chrome

If you open the Android Settings app, select Applications and choose Firefox, do you have it set to install unknown apps?

Yes I've done that. Actually the problem is that when I am downloading a file, and then go to see this file to Firefox menu, then Tools>downloads, when I click on the file, it doesn't open

Does the same thing happen in our new browser, Firefox Preview?

I don' use Firefox preview because there are not many ad-ons