-Some Spoilers for The Last Hope-
After Firestar became leader of ThunderClan, his personality and character itself kind of took a weird toll. Suddenly, he is avoiding fights at all costs, which I find odd because he caused a few (yet stopped a few) in his youth. He is also always right, it seems, compared to the other leaders, let alone cats. Next, he is all of the sudden a leader who acts like he knows everything. Lastly, he appears to know and willingly take on the task of being the Fourth cat when discussing it with Dovewing.
Did he suddenly gain these personalities after StarClan granted him nine lives? Or was the loss of focus on his basic leadership life for the Erins the reason he suddenly gained power?