The theory was largely that while firearms weren't all that effective in battle compared to highly trained soldiers with other weapons, firearms and ammunition were enormously expensive and required little training to use.
This meant that instead of having to train your populous with weapons that were easily reproducible, you could give your untrained levies firearms for war and then take them back again as they left the army.
The expense actually came as a plus as it meant that only a central government with the power to raise taxes could effectively wage war. This took power away from everyone but the central government as they didn't have the funds to wage war with firearms.
Cannons were doubly effective in this regard as they were both expensive, and they could be used to level previously near impenetrable castles of disobedient vassals, placing far more authority in the hands of the central government.
Is this a good theory?
I'm guessing that your professor is referring to a simplified version of the argument in Geoffrey Parker's The Military Revolution: Military Innovation and the Rise of the West, 1500-1800. It's a book that was well regarded when it was released and it's still taught in graduate courses on early modern Europe. In the book, Parker does extensively cover the changes in military technology and tactics during this period and how they changed how war was conducted:
So the military revolution of early modern Europe possessed a number of separate facets. First, the improvements in artillery in the fifteenth century, both qualitative and quantitative, eventually transformed fortress design. Second, the increasing reliance on firepower in battle - whether with archers, field artillery or musketeers - led not only to the eclipse of cavalry by infantry in most armies, but to new tactical arrangements that maximized the opportunities of giving fire. Moreover these new ways of warfare were accompanied by a dramatic increase in army size (24).
However, the key to Parker's argument isn't simply the nature of the weapons themselves, but in the ways that these changes in military organization and technology had holistic effects that shaped the form of the state.
Parker argues that one of the reasons Europe was able to become a dominant power on the globe was because during the 16th and 17th centuries, the state had to develop effective and efficient means of gathering resources in order to fuel military ventures, most importantly a growing state bureaucracy that had the capital and the power to effect significant changes. At the time, the only entities able to field enough money to improve infrastructure (e.g. roads, ports, fortifications) and pursue long-term military actions were states that were able to effectively centralize and mobilize resources - whether that was money, materiel, consumer goods, persons, or whatever. Things like banking, global trade, state record-keeping, and public works all fit under this umbrella.
Furthermore, this form of organization, Parker holds, is one of the principal reasons why "the West" was able to dominate other parts of the globe in the 18th and 19th centuries. The interconnected changes in practices of war and state apparatuses, Parker declares, meant that "The tools of empire used to overthrow the Marathas, the Ch'ing [sic] and the Tokugawa were totally different from those used to subjugate the Aztecs or the Incas" (146):
...the Westernization of war also required replication of the economic and social structures and infrastructures, in particular the machinery of resource-mobilization and modern finance, on which the new techniques depended (174).
The book is highly readable and widely available, so I would check it out if you haven't had the chance. It also pairs well with another book in the same vein: John Brewer's The Sinews of Power: War, Money and the English State 1688-1783.