I have a Nuxt project that was working normally before, but Firefox recently stopped loading it locally.

When I open:

http://localhost:3000

Firefox returns:

426 Upgrade Required

I also tried running the project on different ports, but Firefox shows the same issue.

The project works correctly in:

• Chrome

• Safari

• The deployed production environment

This only happens locally and only in Firefox.

In Firefox DevTools, the request is:

GET http://localhost:3000/
Status: 426 Upgrade Required

I have not intentionally configured a proxy, reverse proxy, or WebSocket server for this project.

What could cause Firefox to receive a 426 Upgrade Required response from a Nuxt local development server, while other browsers work normally? Is there a Firefox setting, extension, proxy/VPN configuration, or Nuxt/Vite setting I should check?