https://www.reddit.com/r/FirefoxCSS/comments/1dwi3ct/how_to_curve_the_toolbox_in_firefox_like_this/

gistfile1.txt

#browser, #appcontent, #tabbrowser-tabbox, 
#tabbrowser-tabpanels, .browserSidebarContainer { 
    background: black !important; 
}

#browser, #appcontent, #tabbrowser-tabbox,
#tabbrowser-tabpanels, .browserSidebarContainer, .browserStack,
#tabbrowser-tabpanels browser {
    border-top-left-radius: 15px !important;
    border-top-right-radius: 15px !important;
}

#navigator-toolbox, #nav-bar, #main-window, body {
  background: black !important;

  & #navigator-toolbox {
    border-bottom: 0px !important;
  }
}