I found a Reddit post explaining how to set this up, but it was missing a crucial step, so here we go. This should work on any OS.

First, get into your profile directory. If you don’t know where it lives, go to about:profiles in FF and hit the button that takes you to the root folder.

In there, create a “chrome” folder, and in there, put your image, as well as a userContent.css file. That CSS file should contain the following:

@-moz-document url-prefix(about:home), url-prefix(about:newtab) { .click-target-container *, .top-sites-list * { color: #fff !important ; text-shadow: 2px 2px 2px #000 !important ; }

body { background: url(wallpaper.jpg) !important ; background-size: cover !important ; } }

(Obviously change wallpaper.jpg to your image’s filename.)

Now, before this will work, we need to enable custom CSS. Go to about:config and search for toolkit.legacyUserProfileCustomizations.stylesheets. Set that to “true” and restart FF.

That’s it, new tabs should have a background now.