"ERROR in getInternalNameOfClass() called on a non-ES5 class: expected AngularFireModule to have an inner class declaration"

Try changing target in the compilerOptions of your tsconfig.json from es5 to es2015

I encountered same issue during migration from Angular8 to Angular9.

In Angular9 they introduced new generation compilation and rendering pipeline called Angular Ivy. The documentation says:

Ivy applications can be built with libraries that were created with the View Engine compiler. This compatibility is provided by a tool known as the Angular compatibility compiler (ngcc). CLI commands run ngcc as needed when performing an Angular build.

Source: https://angular.io/guide/ivy

So in order to project with "target": "es5" work, after using npm install you have to run ngcc.

You can add it in your package.json file:

{
  "scripts": {
    "postinstall": "ngcc"
  }
}

Then it should run automaticly after running npm install.