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What’s Java? Android native UI?

It’s for GeckoView, the embedding API for Android. (I used to work on that team)

And why Python? Does Firefox come with a Python interpreter?

Python is for build tooling I think

Correct, the build system is written in Python, with the definitions of rules for each module in files such as https://searchfox.org/mozilla-central/source/toolkit/library...

It’s entirely for build and test. No python actually ships with the browser.