This is how to configure JwtBearer authentication in C# to use a firebase id token

Startup.cs

services
  .AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
  .AddJwtBearer(options =>
  {
      options.Authority = "https://securetoken.google.com/firebase-project";
      options.TokenValidationParameters = new TokenValidationParameters
      {
          ValidateIssuer = true,
          ValidIssuer = "https://securetoken.google.com/firebase-project",
          ValidateAudience = true,
          ValidAudience = "firebase-project",
          ValidateLifetime = true
      };
  });