diff --git a/app/assets/javascripts/lib/utils/common_utils.js b/app/assets/javascripts/lib/utils/common_utils.js
index 01c4b9821d3..8058672eaa9 100644
--- a/app/assets/javascripts/lib/utils/common_utils.js
+++ b/app/assets/javascripts/lib/utils/common_utils.js
@@ -169,7 +169,10 @@
     w.gl.utils.getSelectedFragment = () => {
       const selection = window.getSelection();
       if (selection.rangeCount === 0) return null;
-      const documentFragment = selection.getRangeAt(0).cloneContents();
+      const documentFragment = document.createDocumentFragment();
+      for (let i = 0; i < selection.rangeCount; i += 1) {
+        documentFragment.appendChild(selection.getRangeAt(i).cloneContents());
+      }
       if (documentFragment.textContent.length === 0) return null;
 
       return documentFragment;
diff --git a/changelogs/unreleased/31193-ff-copy.yml b/changelogs/unreleased/31193-ff-copy.yml
new file mode 100644
index 00000000000..4d44d83d458
--- /dev/null
+++ b/changelogs/unreleased/31193-ff-copy.yml
@@ -0,0 +1,4 @@
+---
+title: fix inline diff copy in firefox
+merge_request:
+author: