I'm pretty sure your wildfire is still on the boss and doing damage even if the debuff falls off due to the sheer spam in alliance sized content. An easy way to confirm this for yourself is to watch your emnity bar when it's about to go off - Wildfire is one of the single largest sources of damage in the game so you'll definitely see the bar jump slightly when it triggers.
If you're able to make it out amid all the spell effects, you can see the animation, and sometimes the sound effect going off too.
This isn't really MCH related, but for BRDs for example you can watch the scrolling combat text to confirm if your debuffs on are on the boss. If you have venomous bite and windbite on the boss for example, even if the debuff is invisible, if you iron jaws to refresh them and those debuffs are present on the boss, you'll see "+Venomous Bite" and "+Windbite" text flash and scroll past.
