Update: Thunderbird 15.0

Is there any way to disable this horrendous Australis theme and revert to the classic look?

 

The instant messaging introduced I could have done without

 

I got rid of the Australis tabs by editing my userChrome.css file. If you don't have one currently, create a folder in your Thunderbird profile folder called "chrome". Then create a text file named userChrome.css in the chrome folder with the following code in the userChrome.css file:

Code:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* Begin Disable Australis Tabs */

.tabmail-tab:hover > .tab-stack > .tab-background:not([selected]) {
    background-image: none !important;
}

.tab-background[selected] {
    background-image: none !important;
}

/* All code below ripped from Thunderbird 14 tabmail.css file */

/* Tabmail Tabs */

.tabmail-arrowscrollbox {
  margin-bottom: -1px !important;
 }

.tabmail-tabs[overflow="true"] {
  -moz-padding-start: 0px !important;
}

.tabmail-tabs > .tabmail-tab:first-child > .tab-close-button {
  visibility: collapse !important;
}

#tabpanelcontainer:-moz-lwtheme {
  color: inherit !important;
}

/* Tab */

.tabmail-tab {
  -moz-appearance: none !important;
  background: -moz-linear-gradient(bottom, rgba(10%, 10%, 10%, .4) 1px,
              transparent 1px), -moz-linear-gradient(transparent,
              hsla(0, 0%, 45%, .1) 1px, hsla(0, 0%, 32%, .2) 80%,
              hsla(0, 0%, 0%, .2)),
              -moz-linear-gradient(-moz-dialog, -moz-dialog) !important;
  background-origin: border-box !important;
  background-position: 1px 2px !important;
  background-size: -moz-calc(100% - 2px) -moz-calc(100% - 2px) !important;
  background-repeat: no-repeat !important;
  margin: 0 !important;
  padding: 1px 0 3px !important;
  -moz-border-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAoAAAAcCAYAAABYvS47AAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAJBJREFUeNpiFBGXYmJgYABhZihGBn+h+B8LkGAFYhUgVgBiPjSFn4D4ARDfAZmka2Ntpb9t80bO1y+ePkfGIDGQHEgNA9DqkKfPnnn9//9fGIiZgJgBikFsYZAcSA3IRHYpSUmQ8e9AbkGyFsR+B5VjZ4IK/gHi/wyY4D9UDuxbRgbCgHFU4ajCUYU0UggQYADV1D8Ak746KgAAAABJRU5ErkJggg==") 4 3 0 / 4px 3px 0 repeat stretch !important;
  height: 26px !important;
  border-radius: 0 !important;
}

.tabmail-tab:not(:-moz-lwtheme):not([selected="true"]):hover {
  background-image: -moz-linear-gradient(bottom, rgba(10%, 10%, 10%, .4) 1px,
                    transparent 1px), -moz-linear-gradient(hsla(0, 0%, 100%, .3) 1px,
                    hsla(0, 0%, 75%, .2) 80%, hsla(0, 0%, 60%, .2)),
                    -moz-linear-gradient(-moz-dialog, -moz-dialog) !important;
}

.tabmail-tab[selected="true"]:not(:-moz-lwtheme) {
  background-image: -moz-linear-gradient(rgba(255, 255, 255, .7),
                    rgba(255, 255, 255, .5) 30%),
                    -moz-linear-gradient(-moz-dialog, -moz-dialog) !important;
  color: -moz-dialogText !important;
}

.tabmail-tab:-moz-lwtheme {
  color: inherit !important;
}

.tabmail-tab[selected="true"]:-moz-lwtheme {
  background-image: -moz-linear-gradient(rgba(255, 255, 255, .7),
                    rgba(255, 255, 255, .5) 50%) !important;
}

.tabmail-tab:-moz-lwtheme-brighttext:not([selected="true"]) {
  background-image: -moz-linear-gradient(hsla(0, 0%, 40%, .6),
                    hsla(0, 0%, 30%, .6) 80%) !important;
}

.tabmail-tab:-moz-lwtheme-brighttext:not([selected="true"]):hover {
  background-image: -moz-linear-gradient(hsla(0, 0%, 60%, .6),
                    hsla(0, 0%, 45%, .6) 80%) !important;
}

.tabmail-tab:-moz-lwtheme-darktext:not([selected="true"]) {
  background-image: -moz-linear-gradient(hsla(0, 0%, 60%, .5),
                    hsla(0, 0%, 45%, .5) 80%) !important;
}

.tabmail-tab:-moz-lwtheme-darktext:not([selected="true"]):hover {
  background-image: -moz-linear-gradient(hsla(0, 0%, 80%, .5),
                    hsla(0, 0%, 60%, .5) 80%) !important;
}

.tabmail-tab > .tab-image-middle {
  padding-top: 1px !important;
  -moz-padding-start: 1px !important;
}

.tabmail-tab[selected="true"] > .tab-image-middle > .tab-text {
  opacity: 1.0 !important;
}

.tabmail-arrowscrollbox > .scrollbutton-up:-moz-lwtheme-brighttext,
.tabmail-arrowscrollbox > .scrollbutton-down:-moz-lwtheme-brighttext,
.tabs-alltabs-button:-moz-lwtheme-brighttext,
.tabs-closebutton:-moz-lwtheme-brighttext {
  background-color: rgba(0,0,0,.5) !important;
}

.tabmail-arrowscrollbox > .scrollbutton-up:-moz-lwtheme-darktext,
.tabmail-arrowscrollbox > .scrollbutton-down:-moz-lwtheme-darktext,
.tabs-alltabs-button:-moz-lwtheme-darktext,
.tabs-closebutton:-moz-lwtheme-darktext {
  background-color: rgba(255,255,255,.5) !important;
}

.tabmail-tabs:-moz-lwtheme {
  padding-top: 1px !important;
  background-color: transparent !important;
  background-image: none !important;
}

tabpanels {
  -moz-appearance: none !important;
  background-color: transparent !important;
}

.tabs-left,
.tabs-right {
  width: 3px !important;
  border-bottom-width: 0px !important;
}

/* tabmail-tab focus ring */
.tabmail-tab >  .tab-image-middle > .tab-text {
  border: 1px dotted transparent !important;
}

.tabmail-tab:focus > .tab-image-middle > .tab-text {
  border: 1px dotted -moz-DialogText !important;
}

/* Tab DnD indicator */
.tab-drop-indicator {
  list-style-image: url("data:image/png;base64,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");
  margin-bottom: -11px !important;
}

/* close buttons */

.tabs-closebutton-box {
  margin-top: 2px !important;
  margin-bottom: 4px !important;
}

.tab-close-button > .toolbarbutton-icon {
  -moz-margin-end: 0px !important;
  margin-top: 1px !important;
}

.tab-close-button,
.tabs-closebutton {
  -moz-appearance: none !important;
  -moz-image-region: rect(0, 64px, 16px, 48px) !important;
  border: none !important;
  padding: 0px !important;
  cursor: default !important;
  list-style-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAEAAAAAQCAYAAACm53kpAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAB01JREFUeNqsV31MVWUYf8/H/YYQuIh8Kl6mFuaazGyTWgnozE0Wgrqpa4qpy7Vpm63RKmuTGAXOjBZF8o/a/DZNt0D7MMypIzfCBgYCwr3zisrH/b7nnnN6nsN76XC8F+9WL3t2Dud93vf+ns/39zKyLDNkfER7hocc5cmCGEAs9MmSqYcEEgDx0Cfuowcx0yfzhPWoHwTx0qcUBS+JYR/CUAdMJWHlSILDVF9fP6epqWlmX18fMZvNRK/XMyzLMhzHKQqiKBJJkuRgMCh7vV4ya9YssmnTpv4dO3bcxn3eXV9e0n3j2ntswJunYxjCRjFFgl8UZJlIBvOt3EWL91YfPn4GPvs0xkd6lyMEUnlHB7AqY1mNaCOnlvBG1tzc3BfAuGGr1epNTExkLRYLazKZwH6OoQ6QfT6f6PF4pOHhYenBgwdmcFJid3f3Vdxrdc6MrlQdSYrjOYIO4KkHWIahiOFPHndAQJKIOyQSp0Aeney9NwemH2kMjiWDJ5zAq5TRYE4jag+KKiGq1DMEAoHR7OzsRzabTUxPT+enTZvGgwN4zAJFEaIPDgiNjIyEHA5HqKenx3/37t1w6UhmIiYl6AzExLFEDz7jwfBx2xkFgEwhiPCiFxmiY1niEvxJtGS0xj8pgyc5hNdEH43WUYnkACGCFxmIuJCWliaj8ampqQZwgN5oNOp4nldqIBQKiX6/XzAYDFizBMpAevjwoRDeXw8GofFGniVDgRAZDggkxaQnGWa9omD3Bcl9EKtBR5JBWEYiRm5SgjIRsjhSBkiqwCnfWI3xfE5OznaLJe7XvLy8CvjfiILv+A3nqNM49Q9B3XMJCQl8cnKy7vCR7za//8GH5xq+/mZDUlKSGQXf8RvOoQ7q4poweD0kChrPQdi3Nhzqbux2kqKPah39ngDp8wRJ4Z5aR+PfTlIBc1giBjAesyBCvbOAMRewFgLm2eFMxnf8hnNa5/AaD7K9vb2XytesTb9w4fzyhQsXKlHu6+9f/urKlRePHzt6SbVBODsZiDTBuoeo8/VfHLj8VUNjRmvrlRW1+/YrRtrt95YVFy9v3b5ty+Xr16/zoCvjmjAQjh2vezTs9M5NuSs+bWgrLF2TD6EaxHl4z+z4ubntx3e258fpWBIQpYk+oa13wO8E/CbAnwb4CcWfBvjvAX6nZo3MR0gjOygeX79hg3jixMnncKKsbHXb4UOHTuGcprYmBjY8rHmo7fu7dr511mwyMVeuXi3AuWXFRX9s27blbFdX1316Okxazyqdn1FKwQCZcH739vyxPXXtxaVrFuD8lR/OtLd+/Hb+dCNUJnRDQfq3QUbA7wX8A4BfBvyJFP8jwD9Ij0518Agf5Ywc9HoDY9bUtPiQIJAxl2uM0GjEMoaGhpz+YMBjTZkRFGWReH0+D36Lpo9oMP0ZdlxGgn7i9vvjw/MeeB8LiiTNqIdzi4GMkQk3NduANhMQAL9A8QvU+McGr2kSSqNYmP/8ulGXt2Dt2nU3pVCIHD12tGD+swv8HX+2N0XopuGzXsZuD0+puqZ21ZjHt6TolZdvinBsNTc3L7n7SbXrjS0VR6jOpPWM0vUZxRP9rgAp3lPnKCwtz2n/qfmGkkFl6xbxLOdo3bs7PdtiJI+X/+TGDPhnAv4UwD9M8acAfgnw34l2DE4YP3fu0xv9gli2qqTkt7qaqu+VCHjd/OlTp8tmz7aF7tzp+VqziQxdnuAZD50+tP/z+jK3119SVFR4dW35a+dQwe1yyS0Xm0vq9u0PlpeVHkRdXDPBxhiikB8ZUvv1Awd7bC8utf31S0tbS+WORTjHV3/ZtrS0PH/m9OSe1so3bSx5jCzJYY8A/lmAPwvwDwF+O8XPAv4swC8B/h4tE+TUpwCIDSQHxKFK+0yQdOwxILhBSMUH0ufNm5c5f/78ESBEbHFxcU5GRkYWnPkPBgYGhlAhKysrBY5Gq91uH2hpaekFAiR1dHRM6+zsHMB9dj2TMTjnKTMx6tkJtmcEFmnRcUqD9AgiCULjwxMAu4cPiFDnsJd8dmsQMd3THINxVHyqtEeabcJYUJngBbw6+tQoNLBPwwO6QbpUREjNBJHkMFDjUnx8vIgGgrEDaiIE5McFOreRCDmdTgF0GVwT3kMEq4EsEw5iEQdG67jxksDlClsy8AobDEHl+EIS6CMAmUSht256z1A3axfIWCQqz2tIAtE4IxYqHAACZACj0GGiy+WS4JgT8U6g0+kUBQEaEXQlhQqj8aiLa+hlSArqTaNuQUrQcxJhJewFrBJpNFoCG8JUWADLfXCvcEP5BHSmUXoZIlH4fsxUWNYYGXZArJchd01NjbGysjIBjkDP6OgoA9FHbqAIZYLYJzDqHMzLmZmZlqqqKiONlPzSxorG3w9/u3XE5Ys30fsAR7MgnCYyHn+QAX5IfxdrcBVs2NxIU1ye4tIT02Xo/7gOT8f+Q2stluusl5bUffo/9pjFtE5jGVjf12iP+k/X4X8EGAALHaKqr2B7zAAAAABJRU5ErkJggg==");
}

.tabs-closebutton {
  padding: 0 2px !important;
}

.tabmail-tab:not([selected="true"]) > .tab-close-button {
  margin-bottom: 1px !important;
}

.tabmail-tab:not([selected="true"]) > .tab-close-button > .toolbarbutton-icon {
  margin-top: 2px !important;
}

.tab-close-button:hover,
.tabs-closebutton:hover,
.tabmail-tab[selected="true"] > .tab-close-button:hover {
  -moz-image-region: rect(0, 32px, 16px, 16px) !important;
  opacity: 1.0 !important;
}

.tab-close-button:hover:active,
.tabs-closebutton:hover:active,
.tabmail-tab[selected="true"] > .tab-close-button:hover:active {
  -moz-image-region: rect(0, 48px, 16px, 32px) !important;
  opacity: 1.0 !important;
}

.tabmail-tab[selected="true"] > .tab-close-button {
  -moz-image-region: rect(0px, 16px, 16px, 0px) !important;
  /* Make this button focusable so clicking on it will not focus the tab while
     it's getting closed */
  -moz-user-focus: normal !important;
}

.tab-close-button:focus,
.tabs-closebutton:focus {
  outline: none !important;
}

/* Tab Scrollbox Arrow Buttons */

.tabmail-tabs[overflow="true"] .tabs-left {
  display: none !important;
}

.tabmail-arrowscrollbox > .scrollbutton-up {
  margin-top: 1px !important;
  margin-bottom: 1px !important;
  padding: 0 0 3px !important;
  width: 18px !important;
  height: 23px !important;
  -moz-margin-start: 0px !important;
  -moz-margin-end: 2px !important;
  list-style-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAA8AAAARCAMAAAA11AaTAAAARVBMVEXc5+8LEhcSGx4WHiEeJismKCkrMzgsNDovNzwvOD0wOj8yOj41Oj48Q0ZCSUxOVFhRV1xSWFxZYGRdYmddY2dgZ2piam8XHZsCAAAAAXRSTlMAQObYZgAAADdJREFUeNpjYKASYGVF5TLyoHCZ+FG4zIIoXBYhFM283AKohgtzcKIKiHKhCYixs6EKiPCR5Q0AgiABIJWnoDMAAAAASUVORK5CYII=");
}

.tabmail-arrowscrollbox > .scrollbutton-up > .toolbarbutton-icon {
  margin: 2px 0px 0px !important;
}

.tabmail-arrowscrollbox > .scrollbutton-down-stack {
  height: 23px !important;
  margin-top: 1px !important;
  margin-bottom: 1px !important;
  -moz-margin-start: 2px !important;
}

.tabmail-arrowscrollbox > stack > .scrollbutton-down-box {
  width: 18px !important;
}

.tabmail-arrowscrollbox > stack > .scrollbutton-down {
  width: 18px !important;
  margin: 0px !important;
  padding: 0 0 3px !important;
  list-style-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAA8AAAARCAMAAAA11AaTAAAARVBMVEXc5+8LEhcSGx4WHiEeJismKCkrMzgsNDovNzwvOD0wOj8yOj41Oj48Q0ZCSUxOVFhRV1xSWFxZYGRdYmddY2dgZ2piam8XHZsCAAAAAXRSTlMAQObYZgAAADdJREFUeNpjYKASYGVF5TLyoHCZ+FG4zIIoXBYhFM283AKohgtzcKIKiHKhCYixs6EKiPCR5Q0AgiABIJWnoDMAAAAASUVORK5CYII=");
}

.tabmail-arrowscrollbox > stack > .scrollbutton-down > .toolbarbutton-icon {
  margin-top: 2px !important;
  margin-bottom: 0px !important;
  -moz-margin-start: 1px !important;
  -moz-margin-end: 0px !important;
}

.tabmail-arrowscrollbox > .scrollbutton-up:-moz-lwtheme-brighttext,
.tabmail-arrowscrollbox > stack > .scrollbutton-down:-moz-lwtheme-brighttext {
  list-style-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAA8AAAARCAYAAAACCvahAAAAYklEQVR42mNgGMzACYjroTRJwB+I+x48ePAaSLeSYgBY47t3777+BwKoAfUkawTRID4xNuPS6E8zjSDQg6axn1iN2DTPBOJ4skIYBKAG+JNlAKn+piiqKE4kFCdPijMGXgAA9W+O8fSlrHEAAAAASUVORK5CYII=");
}

.tabmail-arrowscrollbox > .scrollbutton-up[disabled],
.tabmail-arrowscrollbox > stack > .scrollbutton-down[disabled] {
  opacity: .4 !important;
}

.tabmail-arrowscrollbox > .scrollbutton-up:-moz-locale-dir(rtl),
.tabmail-arrowscrollbox > stack > .scrollbutton-down:-moz-locale-dir(ltr) {
  -moz-transform: scaleX(-1) !important;
}

If this doesn't work for you, I can post the full code I ripped from Thunderbird 14. I deleted the things that didn't seem to make a difference for me.
Hopefully someone will create a non-Australis classic theme soon. The new tabs were the big thing I didn't like, so hopefully this tweak will do the trick for you too.

 

There have been reports of themes corrupting the installation. Best bet would be to report your issue to Mozilla. Best of luck !

 

Oh yes, please do!

Some interesting developments here:

http://forums.mozillazine.org/viewtopic.php?f=39&t=2531477&p=12251519#p12251519

 

The things I didn't include were the code for the busy throbber and other stuff that didn't affect the tab shape itself. I'm just hacking up what is inside the version 14 installer tabmail.css file. That isn't going to get rid of the entire Australis theme for you.

 

Reclaim your Inbox in your language: https://www.mozilla.org/en-US/thunderbird/all.html

 

Still annoyed with the integrated IM & Chat:
https://support.mozillamessaging.com/en-US/kb/instant-messaging-and-chat