[firefox] The character encoding of the plain text document was not declared - mootool script

I got this error using Spring Boot (in Mozilla),

because I was just testing some basic controller -> service -> repository communication by directly returning some entities from the database to the browser (as JSON).

I forgot to put data in the database, so my method wasn't returning anything... and I got this error.

Now that I put some data in my db, it works fine (the error is removed). :D