I’ve just received my new Fire HD 8 and installed the F-Droid app. I then used it to download Fennec (the code name for Firefox on Android). It works perfectly! I installed the UBlock origin add-on and that works too.
I haven’t connected my Kindle to Amazon and I don’t get any ads from there either. I didn’t buy it from Amazon but Currys (a UK store). I’m concerned that if I connect to Amazon I’ll start getting ads. Can anybody confirm that this is or isn’t the case?