My personal favourites are wallhaven for desktop wallpapers and Walli for mobile ones. I also like Wallpaper Flare for desktop and sometimes Unsplash for both desktop and mobile.
A possible reason not to use it, for now, is the lack of a way to force dark mode on all webpages, like almost all other browsers allow you to do.
It will be possible in the future with full WebExtension support, using Dark Reader for example, but it will be most likely worse than a native implementation, both from a quality standpoint (white flashes when initially loading webpages) and from a performance one, like it is the case with Fennec (the "old" Firefox for Android) and even with the desktop version.
Samsung Internet and Opera have the perfect native implementation of it (and in particular, the Samsung Internet one is a work of art), while Chromium based browsers allow you to do it with a flag (chrome://flags/#enable-force-dark), but I am not a fan of their approach. Notably, you can't enable this flag in the new Edge for Android, even if it is Chromium based, since it does not give you access to chrome://flags.
Here is the relevant GitHub issue requesting a proper way to implement it on Fenix/Firefox Preview.
Could I ask you what theme are you using?
Thanks a lot, I cannot wait to try it :)
Do you know if "Fenix" will support add-ons? I know that geckoview is pretty much gecko, so in theory it should be possible, but neither Focus nor the Android Components reference browser support them yet.
Thanks for this recommendation, it seems quite interesting!