For now, Firefox for Android has only very few UI customization options: you can select dark or light theme, set toolbar to top or bottom, change new tab wallpaper to one from 5 available, toggle a few features and that's it.
I suggest adding a few more options for interface customization, like the ones below. It will make the user experience better and improve the general browser look & feel. I tried to concentrate on the features that globally improve the browser appearance, but (theoretically) won't be too hard to implement.
Add an option to change the purple accent colour to any other one. There will be only a few templates available by default, but it'll be possible to use a custom colour accent as well.
The browser automatically creates colours for lighter and darker accent variants, but the chosen colour itself stays the same both in light and dark mode.
The only exception is the "Default" colour, which uses current light and dark colour palettes and purple-to-yellow gradient for the loading bar.
New tab wallpaper is a very neat option, but it still only provides a choice between 5 built-in images. So, why don't allow users to finally use their own pictures?
The wallpaper itself gets a little whitened/darkened (depending on the light/dark mode being used) to look less intrusive. (UPD: looking back at this concept, I think there also should be an option to additionally dim/blur custom wallpapers if a user thinks they look too noisy)
To look better with wallpaper applied, the new tab page itself needs to be a little refined too: I used the monochrome version of the logo and added small shadows for the standalone text for better readability.
So, what do you think 😃? As I said before, of course these features don't add that much customization options as some "advanced users" might want, but there will be very pleasant features for people who want to add their Firefox a little more personality.