How do I customize Mozilla sync in Firefox iOS

I don't think the app store provided version can be configured such way, as the token origin would be verified on the rust side of code that's built into the official release builds solid.

(I'll check if it's available in the non–release versions later; however IIRC it can only switch between different environments, but does not offer setting any arbitrary endpoint — that might be for forks to customize the connecting service if they're so inclined. I'll see if I find anything relevant.)