Yup, as a fire mage, I found that the greatest way of being defensive (keeping the mob away from you), is all out offense .. kill him before he gets to you. For that reason, I've stacked +fire and spell damage where I can (sitting at 205 fire damage now at level 46)
Important abilities include Flame throwing (those extra 6 yards can make all the diffs), impact (hell, the mob just stands there waiting to be hit again), Blast wave (Stuns all mobs in range). Further up the tree is DragonsBreath which also does a stun type effect. All of these things give you the little extra time you need to nuke away.
ofc, never forget sheep, blink and most important of all (even for a fire mage), Frost Nova. Ice armor can also be your friend if they DO get to use, as it slows them down as well. Although once you learn to use your abilities effectively to make sure you don't get hit, you might switch to Mage Armour to keep mana coming more steadily.
Clever use of sheep, frost nova and all out nuking can allow you to take down 3 mobs at a time (sometimes 4). Not quite as impessive as frost (where you can take 8-10 down at will), but the absolute satisfaction of practically one shotting a mob of your own level if your pyro crits big enough (taking into consideration the pyro DoT and ignite DoT), is unparalled ;-)