If I were in your position I've never should updated.

However since I myself is buy an 99% shure that your device is rooted. So what happend me was that it ruined my device completly( couldn't even start it up).

So what my point is that there is an huge difference between an : Clean Install( re- install/ NEW), and Update.

Since an Update only ads something new,but never removes an already exsisting issue. Since you'll never can change an already factory rooted device- The only thing you can do is to use it as it is ( and perform factory resets= either hard or soft). And when you have used it for 1year or so( or when you have charged your device 5-600 times)- Replace it with a new device.

But as out of my own personal knowledge Samsungs A- series are just completly junk( rubbish). However I myself had a couple of units( one smartphone,one tab).

The basic difference is that you have; 1.A larger screen 2.Larger battery But the downside is that not all apps are compatibel with tablets.

I don't see "Firefox" on that site: https://search.f-droid.org/?q=firefox&lang=en

Can you run the version from Google Play, or are you trying to de-Google your phone?