Since its not available on google play, you can go to your settings and tap app permissions. There you can tap the browser and turn on installing unknown apps. If you did this, press this link and search in Pale Moon. There you can download and install it.

Pale Moon supports complete themes, something which Firefox used to have before version 57.

So we can verify that the browser is not spyware.

Pale Moon supports your GTK theme while Firefox does not.

Pale Moon is an independent fork of an older version of Firefox. Therefore, it is independent from Mozilla and are not affected by their terrible decisions such as removing XUL, adding telemetry, pocket, etc.

"Classic" Firefox add-ons can work, but they are not supported and should be updated or forked to become a Pale Moon add-on.

Unlike most other browsers, Pale Moon uses its custom engine.

Contains much less spyware than Chrome and Firefox and all of it can easily be disabled.