diff --git a/static/js/theme.js b/static/js/theme.js
index 77cea128b9..b51fc3294b 100644
--- a/static/js/theme.js
+++ b/static/js/theme.js
@@ -613,6 +613,44 @@ function initCodeClipboard(){
         return actionMsg;
     }
 
+    document.addEventListener( 'copy', function( ev ){
+        // shabby FF generates empty lines on cursor selection that we need to filter out; see #925
+        var selection = document.getSelection();
+        var node = selection.anchorNode;
+
+        // in case of GC, it works without this handler;
+        // instead GC fails if this handler is active, because it still contains
+        // the line number nodes with class 'ln' in the selection, although
+        // they are flagged with 'user-select: none;' see https://issues.chromium.org/issues/41393366;
+        // so in case of GC we don't want to do anything and bail out early in below code
+        function selectionContainsLnClass( selection ) {
+            for (var i = 0; i < selection.rangeCount; i++) {
+                var range = selection.getRangeAt(i);
+                var fragment = range.cloneContents();
+                if (fragment.querySelector('.ln')) {
+                    return true;
+                }
+            }
+            return false;
+        }
+
+        if( !selectionContainsLnClass( selection ) ){
+            while( node ){
+                // selection could start in a text node, so account for this as it
+                // obviously does not support `classList`
+                if( node.nodeType === Node.ELEMENT_NODE && node.classList.contains( 'highlight' ) ){
+                    // only do this if we are inside of a code highlight node;
+                    // now fix FFs selection by calculating the text ourself
+                    var text = selection.toString();
+                    ev.clipboardData.setData( 'text/plain', text );
+                    ev.preventDefault();
+                    break;
+                }
+                node = node.parentNode;
+            }
+        }
+    });
+
     var codeElements = document.querySelectorAll( 'code' );
     for( var i = 0; i < codeElements.length; i++ ){
         var code = codeElements[i];