I had to look it up, here's what I found (please correct me if I got it wrong):

To change the user agent in Firefox, you can use the built-in Developer Tools. Here's how you can do it:

1. Open Firefox.
2. Press Ctrl + Shift + I on Windows or Cmd + Option + I on macOS to open the Developer Tools.
3. Click on the "Network" tab.
4. Look for a small icon that looks like a mobile phone and a tablet together, usually located at the top-right of the Network tab. This is the "Responsive Design Mode" button. Click on it.
5. Once in Responsive Design Mode, you'll see a dropdown menu at the top of the screen where you can select different user agents (like various mobile devices, different browsers, etc.).

Remember, changing the user agent can sometimes lead to unexpected behavior on websites, as it tells the website that you're using a different browser or device than you actually are. This is usually used for testing and development purposes.

Edit: a word

Sure, but the point is that you shouldn't have to. There should be cross-compatibility.