From the browser Settings -> Site Settings -> Always request desktop site (on/off)

This affects Firefox versions since 133.0.0, including the Mull and Fennec forks.

Relevant link: https://connect.mozilla.org/t5/discussions/desktop-mode-as-default-in-android-was-handled-badly/td-p/79962

Hopefully I can save you the 10min of frustration I just had 😄 I also just discovered that Android 13 and above cannot downgrade an app anymore without uninstalling first…first world problems, I know.