fixes #9538: prevent Firefox rendering bug via some border-fu

dist/css/bootstrap.css

@@ -2473,6 +2473,7 @@ textarea.input-group-sm > .input-group-btn > .btn {
   vertical-align: middle;
   border-top: 4px solid #000000;
   border-right: 4px solid transparent;
+  border-bottom: 0 dotted;
   border-left: 4px solid transparent;
   content: "";
 }
@@ -2591,7 +2592,7 @@ textarea.input-group-sm > .input-group-btn > .btn {
 
 .dropup .caret,
 .navbar-fixed-bottom .dropdown .caret {
-  border-top: 0;
+  border-top: 0 dotted;
   border-bottom: 4px solid #000000;
   content: "";
 }

less/dropdowns.less

@@ -13,6 +13,9 @@
   border-top:   @caret-width-base solid @dropdown-caret-color;
   border-right: @caret-width-base solid transparent;
   border-left:  @caret-width-base solid transparent;
+  // Firefox fix for https://github.com/twbs/bootstrap/issues/9538. Once fixed,
+  // we can just straight up remove this.
+  border-bottom: 0 dotted;
   content: "";
 }
 
@@ -161,7 +164,9 @@
 .navbar-fixed-bottom .dropdown {
   // Reverse the caret
   .caret {
-    border-top: 0;
+    // Firefox fix for https://github.com/twbs/bootstrap/issues/9538. Once this
+    // gets fixed, restore `border-top: 0;`.
+    border-top: 0 dotted;
     border-bottom: 4px solid @dropdown-caret-color;
     content: "";
   }