diff --git a/skin/classic/treestyletab/metal/tab.css b/skin/classic/treestyletab/metal/tab.css
index 77835a03..56c682f2 100644
--- a/skin/classic/treestyletab/metal/tab.css
+++ b/skin/classic/treestyletab/metal/tab.css
@@ -75,7 +75,7 @@
             .tab-background-start[selected="true"],
             .tab-background-middle[selected="true"],
             .tab-background-end[selected="true"],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-content:not([pinned])[visuallyselected="true"],
             .tab-content[pinned]:not([titlechanged])[visuallyselected="true"],
             .tab-background-start[visuallyselected="true"],
@@ -92,7 +92,7 @@
             .tab-background-start[selected="true"],
             .tab-background-middle[selected="true"],
             .tab-background-end[selected="true"],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-background-start[visuallyselected="true"],
             .tab-background-middle[visuallyselected="true"],
             .tab-background-end[visuallyselected="true"]) {
@@ -221,13 +221,13 @@
   .tabbrowser-tab[visuallyselected]:not(:hover)
   :-moz-any(/* Firefox 51 and later */
             .tab-content[pinned][selected],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-content[pinned][visuallyselected]),
 .tabbrowser-tabs[treestyletab-mode="vertical"]
   .tabbrowser-tab[visuallyselected]:hover
   :-moz-any(/* Firefox 51 and later */
             .tab-content[pinned][selected],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-content[pinned][visuallyselected]) {
 	background: none !important;
 }
@@ -273,13 +273,13 @@
   .tabbrowser-tab[visuallyselected="true"]:not(:hover)
   :-moz-any(/* Firefox 51 and later */
             .tab-content[pinned][selected="true"],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-content[pinned][visuallyselected="true"]),
 .tabbrowser-tabs[treestyletab-mode="vertical"]
   .tabbrowser-tab[visuallyselected="true"]:hover
   :-moz-any(/* Firefox 51 and later */
             .tab-content[pinned][selected="true"],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-content[pinned][visuallyselected="true"]) {
 	line-height: 1 !important;
 	border: 1px solid #666666 !important;
@@ -298,13 +298,13 @@
   .tab-content[selected="true"][pinned], /* Firefox 51 and later */
 .tabbrowser-tabs[treestyletab-mode="vertical"]
   .tabbrowser-tab[visuallyselected="true"]:not(:hover)
-  .tab-content[visuallyselected="true"][pinned], /* old Firefox */
+  .tab-content[visuallyselected="true"][pinned], /* Firefox 50 and older */
 .tabbrowser-tabs[treestyletab-mode="vertical"]
   .tabbrowser-tab[visuallyselected="true"]:hover
   .tab-content[selected="true"][pinned], /* Firefox 51 and later */
 .tabbrowser-tabs[treestyletab-mode="vertical"]
   .tabbrowser-tab[visuallyselected="true"]:hover
-  .tab-content[visuallyselected="true"][pinned] /* old Firefox */ {
+  .tab-content[visuallyselected="true"][pinned] /* Firefox 50 and older */ {
 	background: #9d9d9d !important;
 }
 
@@ -313,13 +313,13 @@
   .tab-content[selected="true"][pinned], /* Firefox 51 and later */
 .tabbrowser-tabs[treestyletab-mode="vertical"]
   .tabbrowser-tab[visuallyselected="true"]:not(:hover)
-  .tab-content[visuallyselected="true"][pinned], /* old Firefox */
+  .tab-content[visuallyselected="true"][pinned], /* Firefox 50 and older */
 .tabbrowser-tabs[treestyletab-mode="vertical"]
   .tabbrowser-tab[visuallyselected="true"]:hover
   .tab-content[selected="true"][pinned], /* Firefox 51 and later */
 .tabbrowser-tabs[treestyletab-mode="vertical"]
   .tabbrowser-tab[visuallyselected="true"]:hover
-  .tab-content[visuallyselected="true"][pinned] /* old Firefox */ {
+  .tab-content[visuallyselected="true"][pinned] /* Firefox 50 and older */ {
 	background-color: #b3b2b3 !important;
 }
 
@@ -389,7 +389,7 @@
 :root:-moz-window-inactive
   .tabbrowser-tabs[treestyletab-mode="vertical"]
   .tabbrowser-tab[visuallyselected="true"]
-  .tab-content[pinned][visuallyselected="true"], /* old Firefox */
+  .tab-content[pinned][visuallyselected="true"], /* Firefox 50 and older */
 :root:-moz-window-inactive
   .tabbrowser-tabs[treestyletab-mode="vertical"]
   .tabbrowser-tab[visuallyselected="true"]:hover
@@ -397,7 +397,7 @@
 :root:-moz-window-inactive
   .tabbrowser-tabs[treestyletab-mode="vertical"]
   .tabbrowser-tab[visuallyselected="true"]:hover
-  .tab-content[pinned][visuallyselected="true"] /* old Firefox */ {
+  .tab-content[pinned][visuallyselected="true"] /* Firefox 50 and older */ {
 	background: #e4e4e4 !important;
 }
 
diff --git a/skin/classic/treestyletab/sidebar/sidebar.css b/skin/classic/treestyletab/sidebar/sidebar.css
index ff7d6bae..b9fe92b3 100644
--- a/skin/classic/treestyletab/sidebar/sidebar.css
+++ b/skin/classic/treestyletab/sidebar/sidebar.css
@@ -409,7 +409,7 @@ tabbrowser[treestyletab-mode="vertical"]
             .tab-content:not([selected="true"]):not([visuallyselected="true"])[pinned]:not([titlechanged]),
             /* Firefox 51 and later */
             .tab-content[selected="true"],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-content[visuallyselected="true"]) {
 	border: 0 none !important;
 	background: transparent !important;
@@ -422,7 +422,7 @@ tabbrowser[treestyletab-mode="vertical"]
             .tab-background-start[selected="true"],
             .tab-background-middle[selected="true"],
             .tab-background-end[selected="true"],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-background-start[visuallyselected="true"],
             .tab-background-middle[visuallyselected="true"],
             .tab-background-end[visuallyselected="true"]) {
@@ -437,7 +437,7 @@ tabbrowser[treestyletab-mode="vertical"]
             .tab-background-start[selected="true"],
             .tab-background-middle[selected="true"],
             .tab-background-end[selected="true"],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-background-start[visuallyselected="true"],
             .tab-background-middle[visuallyselected="true"],
             .tab-background-end[visuallyselected="true"]) {
diff --git a/skin/classic/treestyletab/square/base.css b/skin/classic/treestyletab/square/base.css
index 4978796b..c22ab34f 100644
--- a/skin/classic/treestyletab/square/base.css
+++ b/skin/classic/treestyletab/square/base.css
@@ -184,7 +184,7 @@
   :-moz-any(.tab-content:not([pinned="true"]):not([selected="true"]):not([visuallyselected="true"]),
             /* Firefox 51 and later */
             .tab-content:not([pinned="true"])[selected="true"],
-            /+ old Firefox */
+            /* Firefox 50 and older */
             .tab-content:not([pinned="true"])[visuallyselected="true"]) {
 	border: 0 none !important;
 	background: transparent !important;
@@ -197,7 +197,7 @@
             .tab-background-start[selected="true"],
             .tab-background-middle[selected="true"],
             .tab-background-end[selected="true"],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-background-start[visuallyselected="true"],
             .tab-background-middle[visuallyselected="true"],
             .tab-background-end[visuallyselected="true"]) {
@@ -212,7 +212,7 @@
             .tab-background-start[selected="true"],
             .tab-background-middle[selected="true"],
             .tab-background-end[selected="true"],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-background-start[visuallyselected="true"],
             .tab-background-middle[visuallyselected="true"],
             .tab-background-end[visuallyselected="true"]) {
@@ -320,7 +320,7 @@
   .tabbrowser-tab
   :-moz-any(/* Firefox 51 and later */
             .tab-content[selected="true"]:not([pinned]),
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-content[visuallyselected="true"]:not([pinned])),
 .tabbrowser-tabs:not([treestyletab-tabbar-position="top"])
   .tabbrowser-tab
diff --git a/skin/classic/treestyletab/square/tab-surface.css b/skin/classic/treestyletab/square/tab-surface.css
index f6451cf5..ba8075e4 100644
--- a/skin/classic/treestyletab/square/tab-surface.css
+++ b/skin/classic/treestyletab/square/tab-surface.css
@@ -25,7 +25,7 @@
   .tabbrowser-tab[visuallyselected="true"]
   :-moz-any(/* Firefox 51 and later */
             .tab-background[selected="true"],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-background[visuallyselected="true"]) {
 	background-color: var(--tst-tab-surface-selected) !important;
 	background-image: -moz-linear-gradient(
@@ -54,7 +54,7 @@
   .tabbrowser-tab:hover
   :-moz-any(/* Firefox 51 and later */
             .tab-background[selected="true"],
-            /* old Firefox */
+            /* Firefox 50 and older */
             .tab-background[visuallyselected="true"]) {
 	background-color: var(--tst-tab-surface-selected-hover) !important;
 	background-image: -moz-linear-gradient(