1. about:support
2. 'Profile Folder' » OPEN
3. 'chrome' folder » navigate
4. userChrome.css - edit with your custom CSS, save, and reload Firefox

Changes the background color of inactive tabs so they aren't just random text floating on the ui. No idea why that's the default.

/* Inactive tab background color */
.tab-background:not([selected]) {
  background-color: #333b55 !important;
}

/* Active tab background color */
.tab-background[selected] {
  background-color: #ffffff !important;
}