This has been answered a couple times before - see especially u/WARitter's comments in How Effective Was Plate Armor Against Musketballs, Would A 17th Century Breastplate Have Stopped A Pistol/Musketball?, and Advances in (metal) armor through the Centuries. The answer varies by the period but is usually between "sort of" and "no." Gunpowder occasioned an arms race between firearm manufacturers and armor manufacturers; as gunpowder weapons improved (better muzzle velocity, accuracy, weight/composition/design of the bullet), armor struggled to keep up - the best they could generally do in the pre-modern period was increase the thickness of the plate armor, so that it would stop a bullet at range. Eventually, that became too expensive and cumbersome to be effective, and armor played less and less a part in European warfare from about the 16th century onwards. This is not to say that armor ever truly ceased - helmets, cavalry cuirasses, etc. continued in use for centuries, and even during the American Civil War many soldiers tried various armor vests and the like. During WWI serious research went into armor production, bringing modern metallurgy to the task of finding better steel alloys (and/or alternatives, like silk) and finding effective and reproducible designs, but a lot of the same problems cropped up: the armor was generally too heavy and cumbersome for the amount of protection it offered.

So yes, plate armor could stop some firearms at range - depending on the firearm, the thickness of the plate, etc. Even Ned Kelly's improvised armor was effective against civilian firearms. Military firearms, however, quickly outstripped protective capabilities, however, especially at short range, and that continued until pretty much the later 1960s/early 1970s.