I’m Kaiser, one of Method’s Mage players.
Fire Mage currently excels at two things; they're really strong at doing single target damage and are also one of the best classes when it comes to cleaving down targets evenly. In addition to that, fire mage has great mobility and a really strong (if not the best) defensive toolkit.
Fire's greatest weakness is when it comes to consistently doing priority damage while cleaving at the same time because ignite is not as strong as it used to be and you have to spread it manually now.
What has changed
The biggest change is how Fire Mage's mastery works. In Shadowlands, ignite spreading is not a part of Mastery. Instead, you have to use Phoenix Flames in order to spread it.