Rogue is a great class, thats why there will be a lot of rogues running around, it will make it harder for you to get a raidspot, especially as a beginner. Firemage is kinda bad for quite a while, some bosses have fire resistance, so again, you will not have a raidspot for some time as a firemage, go with frost for leveling and raiding untill u encounter bosses that are vulnerable or not resistant to fire (i forgot which raid has that boss, others will tell you). Warlock is great, but you will have to manage your soul stones properly to give your raid healthstones and such.
I would recommend a mage, but a rogue is a great choice aswell, mage is just good overall in every stage of the game (leveling, raiding, pvp etc).