I think allowing users to set images that's in their camera roll as the background in FireFox Mobile would really enhance personalization and customization. It'd also be another unique feature as not many mobile browsers have it (e.g. only Opera, Microsoft Edge have it).
You can still keep the given themes while allowing users to add their own.
I personally think this would allow users to feel like their mobile browser (firefox) is more personal and customized to them.