Iroh: Remember your Breathe of Fire! It could save your life out there!
Zuko: I will. -The Siege of the North, Part 1

When Zuko was in the cooler, he stayed warm using his “breath of fire” technique, thus allowing him to firebend -Avatar Wiki

I hate when people call Zuko a weak firebender or say he was the weakest bender of Team Avatar, because I think people tend to forget that besides mastering Lightning Redirection, he was also able to bend and survive in extremely cold temperatures. This was thanks to mastering the Breath of Fire technique that Iroh showed him. So Zuko, despite being a late bloomer who was suffering from ptsd, was still able master the techniques that Iroh, a renown master firebender invented. Zuko wasn’t a weak bender, he is proof that what Pakku told Katara was true: You have proven that with fierce determination, passion and hard work, you can accomplish anything. Raw talent alone is not enough.

Zuko might have had raw talent, but because of his fierce determination, passion and hard work learning firebending he survived an explosion on his ship. Beat Zhao (twice).Took out combustion man. Redirect lightning against his father. Took down Azula (before she cheated.). He never let the fact that bending didn’t come as easily to him as it did to others stop him. He didn’t let the fact that he probably had a limited range of vision stop him either. Zuko never let his setbacks in life stop him from becoming a badass firebender. In fact he was one of the most adaptable benders on Team Avatar.

Zuko’s drive to be the best that he could be is what lead him to find out what the true meaning of firebending was, and once again Zuko mastered a new technique. Nothing about him or his firebending was ever less than or weaker than the others. Zuko is a powerful bender because he never gave in and he never gives up.