What Websites Can Be Used to Emulate a Browser?

The most reliable way to emulate a browser is to leverage online browser emulators by TestMu AI - formerly known as LambdaTest, which runs your site on real browsers in the cloud. For quick local previews you can also use your browser's built-in DevTools device mode or a tool like the Responsively App, but, as you will see, they only go so far.

The table below compares the main options:

#	Tool	Type	Best For
1	TestMu AI	Real browsers in the cloud	Testing on real browser and OS behavior before release
2	Browser DevTools Device Mode	Built-in viewport emulator	A fast local check of responsive breakpoints while coding
3	Responsively App	Multi-viewport desktop app	Comparing several screen sizes side by side
4	Firefox Responsive Design Mode	Built-in viewport emulator	Quick responsive checks inside Firefox
5	User-Agent Switcher Extensions	Browser add-on	Spoofing a browser or device identity for a quick look

1. TestMu AI (Formerly LambdaTest)

TestMu AI is a cloud testing platform that works as a full browser emulator online, giving you instant access to 3,000+ browser and OS configurations running as real browsers on cloud real machines, not viewport approximations. You pick a browser, version, and resolution, then test in a live session with full debugging tooling.

This is the key difference from local emulation: because you are running the real browser build on a real OS, engine-specific rendering and compatibility bugs surface instead of being masked by your local engine.

3,000+ real browser and OS combinations: Test current and legacy browser versions on Windows and macOS.
Built-in DevTools: Chrome DevTools and Safari Web Inspector available inside every session.
TestMu AI tunnel: Test locally hosted or pre-production builds securely from the cloud browser.
Geolocation testing: Route the session through 170+ countries for locale-specific testing.
Integrated bug capture: Annotate and push issues to JIRA, Slack, and 65+ tools mid-session.

Best for: Cross-browser and cross-OS compatibility testing on real browsers, including ones you cannot install locally.

Cost: A free plan is available with no local setup. Check out this documentation to get started with real-time testing with TestMu AI.

2. Browser DevTools Device Mode

Every major browser ships a built-in device-emulation mode. In Chrome, open it with Cmd+Opt+M on Mac or Ctrl+Shift+M on Windows, then pick a device preset or enter custom dimensions to see how the layout reflows. It is the fastest way to glance at responsive behavior, but it only resizes your own browser, so it cannot show how a different browser renders the page.

Viewport presets: Switch between common device sizes or enter custom dimensions.
Device conditions: Emulate pixel ratio, touch input, and network throttling.
Engine limit: Runs on your desktop engine, so cross-browser rendering bugs stay hidden.

3. Responsively App

A desktop app that stacks several device viewports side by side with synchronized scrolling and clicking, handy for eyeballing multiple sizes at once during development. Like DevTools, every pane runs on the same local engine, so it shows dimensions rather than how an actual Safari or legacy Chrome would behave.

Side-by-side viewports: Preview many breakpoints in a single window.
Synchronized interaction: Scroll and click once, mirrored across every pane.
Engine limit: Emulates dimensions only, not a different browser engine.

4. Firefox Responsive Design Mode

Firefox's own viewport tool, opened with Ctrl+Shift+M on Windows or Cmd+Opt+M on Mac. It is the same idea as Chrome's device mode, useful if Firefox is your primary development browser, and carries the same engine limitation.

Built into Firefox: No install needed if you already develop in Firefox.
Viewport and DPR control: Resize, set pixel ratio, and simulate touch.
Engine limit: Renders with Firefox's engine only, not other browsers.

5. User-Agent Switcher Extensions

Browser add-ons that change the user-agent string so a site thinks you are on another browser or device. They are handy for testing user-agent-based redirects or content, but they change only the reported identity, not the underlying engine.

Quick UA spoofing: Present as Safari, an older Chrome, or a mobile device.
Redirect testing: Check user-agent-based routing and served content.
Engine limit: Only the identity changes; rendering is still your own browser.

How to Choose the Right Emulation Tool

Match the tool to the question you are actually trying to answer:

"Does my layout reflow correctly?" Any viewport emulator works, DevTools or Responsively App is fastest while you build.
"Does the right content show on this device or browser?" A user-agent switcher or an online checker gives a quick read.
"Does the site actually work in the browsers my users run?" Only a real browser answers this, which is where TestMu AI Real Time Testing comes in.

In practice, use the local tools to sketch responsive layouts during development, then confirm on real browsers with TestMu AI before you ship, so an engine-specific bug never reaches your users.