It’s targeting noob users, who may not know about theme add-ons.

I also assume, they had Windows users in mind…

• …who may be surprised of the customization option. It’s not as common on Windows that applications allow theming.
• …where there is not really a “system theme” to match. It’s a mess anyways, so why not make Firefox look the way you like it?
• …who may spend their entire day just in Firefox. So, Firefox’s theme might effectively be their “system theme”.