Most of the time, if people have no clue about tech and say they want or don't want software X, they have no real reason for it.

I bet they couldn't even tell you what exactly they didn't like. I worked as an IT consultant for years and the amount of people telling me to set up a specific software without even knowing what it does or what features it has is staggering.

I once had a client request I 'set up SAP' without even specifying which of their products. They heard all successful companies used SAP. Turns out they didn't need a year long expensive ERP migration but they used a totally outdated version of their existing ERP solution. After upgrading that and installing a plugin for QR code scanning and generation for easier inventory, their requirements were fulfilled.

In many cases my clients were happier with the alternatives provided after we did a thorough analysis on their requirements. They often saved time and money and at some point realised that there's a reason they asked an IT professional to help them.

If your user has a good reason for not wanting Firefox, install Chromium, set up Edge, go to alternativeto.net and find an obscure alternative like Midori... but you don't even mentioned a real reason or requirements to work with here, so you can literally use any browser. Even firefox. We don't even know if your user cares about privacy.