Finding out what version of Firefox do you've is a simple task and here's how you can do that on PC and Android:
In Android, you can do this task in two ways, and you can pick the one you prefer.
Open Firefox app.
Type "about:firefox" (without double quotes) in the Address Bar of the Firefox and then hit "Enter" in the keyboard.
That's it. You can see your Firefox app's version now.
Now, you can see the version of your Firefox app at the top.
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What Version of Firefox Do I Have?