diff --git a/.changeset/ignore-extension-errors.md b/.changeset/ignore-extension-errors.md
new file mode 100644
index 0000000..6611851
--- /dev/null
+++ b/.changeset/ignore-extension-errors.md
@@ -0,0 +1,5 @@
+---
+'@solidjs/start-devtools': patch
+---
+
+Errors thrown by browser extensions' scripts in the page are no longer listed as the app's errors.
diff --git a/src/dev-toolbar/extension-errors.test.ts b/src/dev-toolbar/extension-errors.test.ts
new file mode 100644
index 0000000..655bd42
--- /dev/null
+++ b/src/dev-toolbar/extension-errors.test.ts
@@ -0,0 +1,39 @@
+import { describe, expect, it } from 'vitest';
+import { isExtensionError } from './extension-errors.js';
+
+function withStack(stack: string): Error {
+  const error = new Error('boom');
+  error.stack = stack;
+  return error;
+}
+
+describe('isExtensionError', () => {
+  it('trusts the file an error event names', () => {
+    expect(isExtensionError(undefined, 'chrome-extension://abc/executors/200.js')).toBe(true);
+    expect(isExtensionError(undefined, 'http://localhost:5173/src/app.tsx')).toBe(false);
+  });
+
+  it('reads the first frame of a V8 stack', () => {
+    const stack = 'TypeError: x\n    at Y (chrome-extension://abc/executors/200.js:1:761)';
+    expect(isExtensionError(withStack(stack))).toBe(true);
+  });
+
+  it('reads the first frame of a Firefox or Safari stack', () => {
+    expect(isExtensionError(withStack('x@moz-extension://123/content.js:3:9'))).toBe(true);
+    expect(isExtensionError(withStack('x@safari-web-extension://123/c.js:1:1'))).toBe(true);
+  });
+
+  it('keeps an app error that passed through an extension further down', () => {
+    const stack = [
+      'Error: boom',
+      '    at Broken (http://localhost:5173/src/app.tsx:12:9)',
+      '    at wrapped (chrome-extension://abc/hook.js:1:1)',
+    ].join('\n');
+    expect(isExtensionError(withStack(stack))).toBe(false);
+  });
+
+  it('keeps values with no stack', () => {
+    expect(isExtensionError('a string')).toBe(false);
+    expect(isExtensionError(new Error('no stack'))).toBe(false);
+  });
+});
diff --git a/src/dev-toolbar/extension-errors.ts b/src/dev-toolbar/extension-errors.ts
new file mode 100644
index 0000000..55afc85
--- /dev/null
+++ b/src/dev-toolbar/extension-errors.ts
@@ -0,0 +1,25 @@
+/** The URL schemes browsers serve extension scripts from. */
+const EXTENSION_URL = /^(?:chrome|moz|safari-web|ms-browser)-extension:\/\//;
+
+/** The URL of the first frame of a stack, the one the value was thrown in. */
+function firstFrameUrl(stack: string): string | undefined {
+  for (const line of stack.split('\n')) {
+    // V8 writes `at fn (url:line:col)`, Firefox and Safari write `fn@url:line:col`.
+    const match = /(?:\(|@|at\s)([a-z][a-z0-9+.-]*:\/\/[^\s)]+)/i.exec(line);
+    if (match) return match[1];
+  }
+  return undefined;
+}
+
+/**
+ * Whether a thrown value came from a browser extension rather than the app.
+ * An extension's content script runs in the page, so its errors reach the
+ * window's `error` and `unhandledrejection` events like the app's do.
+ */
+export function isExtensionError(error: unknown, filename?: string): boolean {
+  if (filename && EXTENSION_URL.test(filename)) return true;
+  const stack = error instanceof Error ? error.stack : undefined;
+  if (!stack) return false;
+  const url = firstFrameUrl(stack);
+  return !!url && EXTENSION_URL.test(url);
+}
diff --git a/src/dev-toolbar/index.tsx b/src/dev-toolbar/index.tsx
index 3465de9..ebff7b2 100644
--- a/src/dev-toolbar/index.tsx
+++ b/src/dev-toolbar/index.tsx
@@ -35,6 +35,7 @@ import { HighlightUpdatesOverlay } from './highlight-updates/overlay.js';
 import { installElementTracking, type InspectedComponent } from './inspector/lookup.js';
 import { InspectorOverlay } from './inspector/overlay.js';
 import { addIssues, captureIssues, type IssueEntry } from './issues/tracker.js';
+import { isExtensionError } from './extension-errors.js';
 import { openInEditor } from './open-in-editor.js';
 import { CopyReportButton } from './report/CopyReportButton.js';
 import { pageRoute } from './report/markdown.js';
@@ -362,10 +363,13 @@ export function DevToolbar(props: DevToolbarProps) {
       if (!error.error && error.message?.startsWith('ResizeObserver loop')) {
         return;
       }
+      // A browser extension's script throws in the page too. It is not the app's.
+      if (isExtensionError(error.error, error.filename)) return;
       pushError(error.error ?? error);
     };
 
     const onRejectionEvent = (event: PromiseRejectionEvent) => {
+      if (isExtensionError(event.reason)) return;
       pushError(event.reason);
     };
 
diff --git a/tests/e2e/devtools.spec.ts b/tests/e2e/devtools.spec.ts
index b88d493..7a5fa8f 100644
--- a/tests/e2e/devtools.spec.ts
+++ b/tests/e2e/devtools.spec.ts
@@ -166,6 +166,49 @@ test('fits the dock and the padding to the window', async ({ page }) => {
   expect(await page.evaluate(() => document.documentElement.style.paddingBottom)).toBe('');
 });
 
+test('leaves out errors thrown by browser extensions', async ({ page }) => {
+  await page.goto('/');
+
+  // An extension's script runs in the page, so its errors reach the window too.
+  await page.evaluate(() => {
+    const error = new TypeError("Cannot read properties of undefined (reading 'M_ID')");
+    error.stack = [
+      "TypeError: Cannot read properties of undefined (reading 'M_ID')",
+      '    at Y (chrome-extension://abcdefghijklmnop/executors/200.js:1:761)',
+      '    at E (chrome-extension://abcdefghijklmnop/executors/200.js:1:1442)',
+    ].join('\n');
+    window.dispatchEvent(
+      new ErrorEvent('error', {
+        error,
+        message: error.message,
+        filename: 'chrome-extension://abcdefghijklmnop/executors/200.js',
+      }),
+    );
+    const rejection = new Error('extension rejection');
+    rejection.stack = 'Error: extension rejection\n    at x (moz-extension://1234/content.js:3:9)';
+    const promise = Promise.reject(rejection);
+    promise.catch(() => {});
+    window.dispatchEvent(
+      new PromiseRejectionEvent('unhandledrejection', { promise, reason: rejection }),
+    );
+  });
+
+  // Neither counts as an error of the app, so the dock stays closed.
+  const launcher = page.getByRole('button', { name: 'Open Start Devtools' });
+  await expect(launcher).toHaveAttribute('aria-label', 'Open Start Devtools');
+  await expect(page.getByRole('region', { name: 'Start Devtools' })).toBeHidden();
+
+  // An error of the app still counts.
+  await page.locator('#throw-client-error').click();
+  await expect(page.getByRole('tab', { name: 'Errors', exact: true })).toHaveAttribute(
+    'aria-selected',
+    'true',
+  );
+  await expect(page.locator('[data-solid-error-viewer-error-info-message]')).toHaveText(
+    'client boom',
+  );
+});
+
 test('captures client errors', async ({ page }) => {
   await page.goto('/');
   await page.locator('#throw-client-error').click();