diff --git a/plugins/prism-line-numbers.css b/plugins/prism-line-numbers.css
index 036d373..3542034 100644
--- a/plugins/prism-line-numbers.css
+++ b/plugins/prism-line-numbers.css
@@ -16,6 +16,20 @@ code-input.line-numbers, .line-numbers code-input {
 }
 
 /* Make keyboard navigation still fill width */
-code-input .code-input_dialog-container .code-input_keyboard-navigation-instructions {
+code-input.line-numbers .code-input_dialog-container .code-input_keyboard-navigation-instructions, .line-numbers code-input .code-input_dialog-container .code-input_keyboard-navigation-instructions {
   width: calc(100% + max(3.8em, var(--padding, 16px)))!important;
 }
+
+/* Override Prism styles so there's no display:inline, relatively-positioned code element which breaks offsetTop, used in FindAndReplace, in Firefox. */
+code-input pre[class*=language-].line-numbers>code {
+  position: static;
+}
+/* Line numbers now positioned relative to the pre element not the code element. */
+code-input .line-numbers .line-numbers-rows {
+  left: 0;
+  top: var(--padding);
+}
+/* Things with padding when instructions are present */
+code-input:not(:has(.code-input_keyboard-navigation-instructions:empty)):has(textarea:focus):not(.code-input_mouse-focused) .line-numbers .line-numbers-rows {
+  top: calc(var(--padding) + 3em);
+}
diff --git a/tests/tester.js b/tests/tester.js
index 2e1b7d8..80b54b3 100644
--- a/tests/tester.js
+++ b/tests/tester.js
@@ -398,7 +398,7 @@ console.log("I've got another line!", 2 < 3, "should be true.");
     textarea.selectionStart = 0;
     textarea.selectionEnd = textarea.value.length;
     backspace(textarea);
-    addText(textarea, "// hello /\\S/g\nhe('llo', /\\s/g);\nhello");
+    addText(textarea, "// hello /\\S/g\nhe('llo', /\\s/g);\nhello\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\na very very very very very very very very very very very very long line with many many many many many many many many many many many words\nline\nline\nline\nline\nline\nline\nline");
     textarea.selectionStart = textarea.selectionEnd = 0; // So focuses on first match
 
     await waitAsync(50); // Wait for highlighting so text updates
@@ -437,7 +437,7 @@ console.log("I've got another line!", 2 < 3, "should be true.");
     replaceInput.value = "do('hello";
     replaceButton.click();
     await waitAsync(50); // Wait for buttons to work
-    assertEqual("FindAndReplace", "Replaces Once Correctly", textarea.value, "// hello /\\S/g\ndo('hello', /\\s/g);\nhello");
+    assertEqual("FindAndReplace", "Replaces Once Correctly", textarea.value, "// hello /\\S/g\ndo('hello', /\\s/g);\nhello\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\na very very very very very very very very very very very very long line with many many many many many many many many many many many words\nline\nline\nline\nline\nline\nline\nline");
     nextMatchButton.click(); // Back to first match
 
     // Exit find input box
@@ -458,7 +458,26 @@ console.log("I've got another line!", 2 < 3, "should be true.");
 
     replaceInput.value = "hi";
     replaceAllButton.click();
-    assertEqual("FindAndReplace", "Replaces All Correctly", textarea.value, "// hi /\\S/g\ndo('hi', /\\s/g);\nhi");
+    assertEqual("FindAndReplace", "Replaces All Correctly", textarea.value, "// hi /\\S/g\ndo('hi', /\\s/g);\nhi\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\nline\na very very very very very very very very very very very very long line with many many many many many many many many many many many words\nline\nline\nline\nline\nline\nline\nline");
+
+    findInput.value = "line";
+    regExpCheckbox.click(); // Now no regex
+    await waitAsync(200); // Wait for value to update
+
+    // Go to "line" on very long line
+    codeInputElement.style = "height: 100px; width: 1000px;";
+    previousMatchButton.click();
+    previousMatchButton.click();
+    previousMatchButton.click();
+    previousMatchButton.click();
+    previousMatchButton.click();
+    previousMatchButton.click();
+    previousMatchButton.click();
+    previousMatchButton.click();
+
+    await waitAsync(200); // Wait for highlighting so matches update
+    testAssertion("FindAndReplace", "Scrolls to Match Correctly", confirm("Is the match on the very long line highlighted orange and near the centre of the element?"), "user-judged");
+    codeInputElement.style = "";
 
     // Exit find input box
     codeInputElement.querySelector(".code-input_find-and-replace_dialog").dispatchEvent(new KeyboardEvent("keydown", { "key": "Escape" }));