Rounded Firefox Tabs (chrome/userChrome.css)

userChrome.css

/** userChrome.css
 * Rounds the browser tabs in firefox
 * toolkit.legacyUserProfileCustomizations.stylesheets must be true in about:config
 * Drop this in ~/.mozilla/firefox/*.default/chrome/userChrome.css
 */

/* Remove colored line above each tab */
.tabbrowser-tab .tab-line {
   visibility: hidden;
}

/* Pointer tab cursor (lol!) */
.tabbrowser-tab * {
  cursor: pointer !important;
}

/* Curve the background tabs */
.tabbrowser-tab,
.tabbrowser-tab .tab-background {
  border-top-left-radius: 6px !important;
  border-top-right-radius: 6px !important;
}

/* Shorten / hide the vertical lines between background tabs */
.tabbrowser-tab::before, .tabbrowser-tab::after {
  display: none !important;
  margin-top: 6px !important;
}

/* Shade and outline the inactive tabs */
.tabbrowser-tab .tab-background {
  background-color: rgba(0, 0, 0, 0.2) !important;
  border: 1px solid rgba(33, 33, 33, 0.4) !important;
}

/* Highlight the selected tab */
.tabbrowser-tab .tab-background[selected="true"] {
  background-color: rgba(185, 130, 181, 0.5) !important;
}

The browser styles can be debugged with the Browser Toolbox.
More info on userChrome.css can be found at https://userchrome.org.