If you're on macOS these solutions may hide the traffic light window buttons. One way to bring them back is to enable the title bar via View > Toolbars > Customize Toolbar… and toggling Title Bar in the bottom left corner.
Alternatively, you can use this CSS rule in userChrome.css
to only hide the tabs and not the window buttons:
#TabsToolbar {
.toolbar-items, .titlebar-spacer {
display: none;
}
}
/* Move private browsing indicator back to right now that tabs are hidden. */
#private-browsing-indicator-with-label {
flex-grow: 1;
justify-content: right;
margin-inline: 0 !important;
> label {
margin-inline-end: 0 !important;
}
}
You may need to adjust the padding a bit to make it consistent with other windows, e.g.:
#TabsToolbar {
padding: 6px 7px 5px 7px !important;
.titlebar-buttonbox {
margin-inline: 0 !important;
}
}
These rules can be previewed with the Browser Toolbox under Tools > Browser Tools > Browser Toolbox. Open the Style Editor pane and make a new style sheet on the top left to apply user chrome styles without restarting Firefox.
The browser DOM can also be shown by navigating to:
view-source:chrome://browser/content/browser.xhtml
userChrome.css
but with different element name and ID.