The phrase "arm firmly" is correct and usable in written English.
It can be used in contexts where you are describing the position or strength of an arm, often in relation to holding or gripping something securely.
Example: "He held the sword with his arm firmly extended, ready for battle."
Alternatives: "arm steady" or "arm strong".
She had his arm firmly in her hand.
Sarah is over his shoulder, kissing his head, popping jelly beans into his mouth, holding his arm firmly.
Coach Hans Backe, arm firmly twisted behind his back, gave da Luz, a San Diego native, his first start in league play.
At one point, an officer emerged from a scrum of people with his arm firmly around the neck of a protester, who appeared to be in distress.
Newspaper photographs caught a seemingly joyful moment: a grinning Sam Hallam – his arm firmly gripped by his mother, Wendy – walking down the court steps doused with the champagne of well-wishers.
But on a dais here after a joint news conference on Monday, the two men, neither known for their social ebullience, were inseparable: the youthful, lanky American president's arm firmly fixed on the older man's shoulder, Mr. Singh grinning widely with his arm wrapped tightly around Mr. Obama's waist.
Wrap your arms firmly around his neck and ride away.. View Article By Rebecca Mead By Anthony Lane By Alan Burdick By George Packer.
She seized our arms firmly and marched us back into the store.
Most turned up open-collared and, when others crossed themselves, the leftists kept their arms firmly to their sides.
With arms firmly crossed over a solid pelvic section supported by strong thighs, she has the generalized look of an ancient Roman goddess.
I was waiting for the guy in the street in the evening when suddenly two big men held my arms firmly and pointed a gun to my head.
