@thomas303 Thank you very much! The system was not broken, because I disabled apparmor for Firefox as you said in the opening post (altough you intended it just for testing). Anyway, your instructions were very complete, and my doubt was mainly compatibility with Ubuntu update. But as you said, "Now that the original profile file remains unchanged, apt-get won't complain when the official package gets updated.", so it's fine for me. And it works like a charm, by the way!
@thomas303 Thank you very much! The system was not broken, because I disabled apparmor for Firefox as you said in the opening post (altough you intended it just for testing). Anyway, your instructions were very complete, and my doubt was mainly compatibility with Ubuntu update. But as you said, "Now that the original profile file remains unchanged, apt-get won't complain when the official package gets updated.", so it's fine for me. And it works like a charm, by the way!