Erm i think it would do just the opposite.
Firearms ended the era of the skill of the individual, Knights and other elite units fell by the wayside because any yokel with working eyes could be a rifleman.
Sure maybe magic could amplify the effects of the firearm in question, but it wouldn't stop a column of soldiers from peppering a mage with a barrage of rifle fire.
Not really. Elite heavy cavalry continued being utilized well into the 18th century. But this is of course aside the point, knights aren't mages. Really, what advantages would early firearms provide over your standard bows & arrows against mages? Better penetration against magical defenses? Offering some form of defense to the civilians who can purchase the guns and the powder? The reasons that firearms became so popular in military engagements in our world don't really apply to fighting mages.
As for the negatives, well, every man operating a firearm is going to be carrying cartridges containing gunpowder and/or a powder horn, while also trying to keep a match cord alight... doesn't sound too great if your opponent can control fire and ice. Not to mention that if you're going to be issuing firearms to Templars, then that also means there will have to be a powder magazine installed at each Circle, providing the means for any pernicious mage to create a magnificent explosion with very little effort needed on their part. I suppose you could move the powder magazines away from the main building for such safety purposes, but that would also diminish the Templars' effectiveness to quickly respond to threats.
Now, what would a firearm do for a mage? Well, most of them wouldn't have to bother with powder, since they are capable of providing the ignition themselves. It would also make a great tool for dealing with heavily armored foes who can shield themselves from magical projectiles, while also being quite cost effective when it comes to mana. Ironically, it would make for quite an ideal Templar killer. Of course, there's also the potential ways in which they could magically enhance the weapon or projectile.