Sadly that's not practical for some sites and services.

I'd love to know what website or service you're referring to. I've been using Firefox since shortly after they released Quantum, and the only issues I've had have been related to things blocked via addons. If I disable all those addons, everything just works. So I'm using two profiles, one that has only got uBlock Origin, and my main one that has many different privacy addons and privacy configurations changed. On the rare occasion a website doesn't work on my main profile, I try it with my secondary profile and it works perfect.