Well, somewhat snarky. It's still a legimate question.

Why would one "need" plugins on a mobile browser? What kind of functionality that mobile Firefox doesn't provide?

>The topic is about owning your own hardware/software combo - so having addons/customization is the definition of it.

Well, the topic is about owing your computer. Which has some merit (even though owing is a kind of a weasel word: you do own it, even if the OS enforces this or that measure. You can sell it at any time, for example, break it and nobody will ask you to return it, etc.).

So, the real topic is "doing whatever you want with your OS, with the ability to disable all checks, protections, etc, install custom everything etc".

Which I can see the appeal in some cases.

For a mobile phone what exactly is the great appeal?

You usually view webpages in a very minimal interface, small screen, often on the go or leisurly, and with limited interaction on a mobile phone. So, aside from something like an adblocker (for which there are solutions), what would one use?

#2 is dark reader.