I know that Firebase doesn't allow you to change the email content for security purposes, but is there a way I could have multiple %APP_NAME%? I tried to setup multiple apps inside my project and I tried to send the email verification from those apps upon registration, but the %APP_NAME% wasn't replaced with the app nickname and it didn't work properly.
EDIT: We have multiple websites that use the same project, consider x and y as different websites and x as the main one: When the user registers through x, they would receive the correct %APP_NAME&, but when they register through y, they are receiving the %APP_NAME% of x, which is annoying because they didn't register through x.
A single Firebase project currently has only one app name, so what you want isn't possible in Firebase Hosting/Authentication. If you want to use a different app name for each web site, you'll need to put each web site in its own Firebase project. If you want to have all web sites in one Firebase project, you'll have to come up with a single app name to cover them all.
Firebase logic typically recommends to use one project per logo or app name. So putting variants of the same app (e.g. multiple languages, or pro and non-pro versions) of an app in a single project works, but putting multiple disjunct apps into a single project doesn't.