The two sides in the Revolutionary War—the Loyalists, who want America to remain a part of the British Empire, and the Patriots, who want America to be an independent country—are constantly at each other's throats. As the hatred and the animosity between the two sides grows ever more intense, it's inevitable that the language each side uses about the other should be less than civilized. In such a fraught environment, colorful figurative language becomes very much the order of the day.
When a mob of Patriots destroys a statue of King George, they plan to melt down the lead to make bullets:
We'll fire Majesty at the redcoats!" joked a man with a booming voice.
These are the words of a Patriot who helped bring the statue crashing to the ground. He's using figurative language here. So when he says that the Patriots are going to "fire Majesty at the redcoats" he doesn't mean it literally. (It was a statue of King George that the Patriots brought down, not the man himself).
The contemptuous use of the word "Majesty" indicates that the Patriots do not respect the British monarchy, nor for those Loyalists who still insist on supporting him. It is ironic indeed that a statue of King George should be melted down for bullets that will then be used against soldiers fighting on his side.