Switching from Chrome to Firefox? Fix Firefox's look & feel with a few lines of CSS

userChrome.css

/* How do you use this? See https://www.userchrome.org/how-create-userchrome-css.html */

/* Hide blue stripe on active tab */
.tab-line[selected="true"] {
  opacity: 0 !important;
}

/* Hide gray stripe on tab being hovered over */
.tabbrowser-tab:hover .tab-line {
  opacity: 0 !important;
}

/*
 * Hide line below container tabs, e.g., Facebook Container
 * 
 * Note: I don't have any use for the color line on a tab indicating which container it is in. (All of my
 * container rules are automated.) I hide the line and check out the address bar if I want to confirm which container I'm in.
 * Someone has pointed out that this may be unexpected, though, so I'm not enabling it by default here.
 */
/* .tab-bottom-line {
  display: none !important;
} */

/* Don't add big blue focus ring to search box when you click into it */
#urlbar:not(.hidden-focus)[focused="true"] {
  box-shadow: none !important;
  border-color: rgba(0, 0, 0, 0.2) !important;
}

/* Hide Page Actions menu inside the address bar */
#pageActionButton {
  display: none !important;
}

/* Hide separator inside address bar */
#urlbar #pageActionSeparator {
  display: none !important;
}

I've commented out the CSS that hides container tab lines, per a request on HN.