glsk

UPD: See this post for better icons and Breeze tab style.

I was upset that close tab (×) and new tab (+) buttons in Firefox didn’t match my current icon theme (Breeze). Thankfully, a workaround is still possible in v.48.

Just add the following snippet to your userChrome.css file in [Firefox_profile_directory]/chrome folder:

.tab-close-button,
.tabs-closebutton,
#find-closebutton {
  list-style-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAMAAAAoLQ9TAAAAY1BMVEUAAACZmZmqqqqSkpKYmJiampqXl5eZmZmampqZmZmYmJiZmZmZmZmZmZmZmZmampqZmZmZmZmYmJiZmZmZmZmZmZmZmZmZmZmZmZmZmZmZmZmZmZmZmZmkpaWlpqa5urzFxshnh6I6AAAAHHRSTlMABQYHKissLY2OkJGTlJaXu7y9vtPU1fLz9Pz9WPW3bAAAAHpJREFUeNpljQkOhCAQBFvXG88VLxzR/7/SGTC7GiuEdDUMQEjVsCxDkcDzqVdymCp03tOPrzQN3Sh5nu/vVsTuPBVDcdwOe22UY3SHh+UlSWMi33in6VW8R4rnoxkS8/g2Aiq6oQCE3d/bANKUxqtR4kKc63nWWST5BJ4TFcJBU3tFAAAAAElFTkSuQmCC") !important;
  margin-right: 0px !important;
}
tab[selected="true"] .tab-close-button,
.tabs-closebutton,
tab:hover .tab-close-button,
#find-closebutton {
  list-style-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAMAAAAoLQ9TAAAAY1BMVEUAAACZmZmqqqqSkpKYmJiampqXl5eZmZmampqZmZmYmJiZmZmZmZmZmZmZmZmampqZmZmZmZmYmJiZmZmZmZmZmZmZmZmZmZmZmZmZmZmZmZmZmZmZmZmxsbGzs7Pd3d329vYe1h82AAAAHHRSTlMABQYHKissLY2OkJGTlJaXu7y9vtPU1fLz9Pz9WPW3bAAAAHpJREFUeNpljQkOhCAQBFvXG88VLxzR/7/SGTC7GiuEdDUMQEjVsCxDkcDzqVdymCp03tOPrzQN3Sh5nu/vVsTuPBVDcdwOe22UY3SHh+UlSWMi33in6VW8R4rnoxkS8/g2Aiq6oQCE3d/bANKUxqtR4kKc63nWWST5BJ4TFcJBU3tFAAAAAElFTkSuQmCC") !important;
}
tab .tab-close-button:hover,
.tabs-closebutton:hover,
#find-closebutton:hover {
  list-style-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAMAAAAoLQ9TAAAAZlBMVEUAAAD/mZnVgIDbkpLngIzngo7ogIvogo7ngIvogYzmgYzmgI3ngI3ngYzngYvngIzmgozngYzngozngYzngYzngYzngYzngYzngYzngYzngYzngYzngYztn6ftoantoqr31dn99PXumoAeAAAAHHRSTlMABQYHKissLY2OkJGTlJaXu7y9vtPU1fLz9Pz9WPW3bAAAAHxJREFUeNpljYkOgyAQRMf7QK1tsSquiP//k90F02p8IWTewAKEQg3zPNQ5Akm3kMe0sfc3/XhJ86ATDc/zfWdFrOOpDIrjum/HRhU+/nC3vCRpjBSa4DTeivtIfX20RG4u36ZASycUgPj59z6CNI0JapS4kFV6mnSZSv4CqK8V1uVZ2DYAAAAASUVORK5CYII=") !important;
}
.tabs-newtab-button {
  list-style-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAMAAAAoLQ9TAAAADFBMVEUAAACAgIBNTU1NTU3Hsk+tAAAAA3RSTlMAAv6SRXlHAAAAG0lEQVR4AWMgHzChCzBTJsCMABgqqCzAiNNTAB/VAEAMg9GLAAAAAElFTkSuQmCC") !important;
}

I encoded images to Base64 on b64.io. Unfortunately, the icons should not be transparent otherwise the default ones will peek through.

To get even closer to native look of the tabs, there’s Squared Australis Tabs addon: