Improved Megumin Red Theme автор Grontface

You can change the toolbar to a transparent red by searching "about:config" and changing these settings (ignore the warning <3):
toolkit.legacyUserProfileCustomizations.stylesheets = false --> true
browser.tabs.allow_transparent_browser = false --> true

Then search "about:support" and find your Profile Folder, open that and create a folder called "chrome", in that make a file called "userChrome.css" (check to see that .txt isn't hidden at the end). Then just paste this in:

#TabsToolbar, #PersonalToolbar {
background: transparent !important;
}
#nav-bar {
background-color: rgba(77, 8, 16, 0.75) !important;
background-image: none !important;
}