Firefox isn't as customizable as other browsers, otherwise forks like Zen Browser wouldn't exist . You may like it more or less, but Mozilla focuses on offering a web browser without frills or extra features that could be considered bloatware . However, they did add a new homepage, a background image for new tabs, and even a weather widget some time ago.
Currently, Firefox offers four images, which isn't a large number, and I think they should soon add the option to add more than one linked gallery. However, this limitation can be overcome by adding them manually. It's a possibility, but for now, it's somewhat hidden. Many future features are activated from about:config , but Firefox already offers Firefox Labs in the stable version, and it's from there that we can add our images for new tabs.
Add your own images to Firefox new tabs
What you see in the header image is not what you currently see in the red panda's browser. To display the box with the plus symbol, indicating that you can "upload" your own image, you have to go to the hamburger menu, Settings, and then to the Firefox Labs section. In the "Customize your browsing" section, you'll find a checkbox that says "Choose a custom wallpaper or color for the new tab." Once you select it, open a new tab and click the gear in the bottom right corner, which represents the new tab settings.
The rest is straightforward: click on the new box, a file manager window will open, and then simply choose the background image you want . Mine isn't very pretty, but I keep it for creating collages, like the one in this article.
It's unknown when the feature will be enabled by default in the stable version of Firefox, but manually enabling it doesn't hurt and offers another customization option.