You don’t need a VPN to spoof the user agent of a web-browser. Quite literally type User Agent Switcher in the Mozilla plugin page and you’ll have a plethora to choose from.

The problem with this is Chromium based browsers work outside the standards for web-development, they implement features/protocols that Firefox or non-chromium based browsers do not comply with as it does not meet the specifications laid out, this is where you get people saying “Use Chrome if Firefox doesn’t work” but ultimately that’s a load of bull shit.

Follow the standards and specifications and your website will work on literally every browser out there, well, except those that are heavily outdated.