。
She was firmly covering her ears.
Literal
She [topic-は] firmly ears [object-を] was-pressing.
しっかり is a 擬態語 ('state mimetic') for firmness, steadiness, or tightness — here describing how she presses her hands against her ears. おさえる (押さえる) is the verb for pressing or holding something down; 耳をおさえる is a stock collocation for covering the ears, typically against unwanted sound or to focus inward. Pairing it with the past continuous ~ていた locks in the image: she was sustaining the gesture, not just doing it briefly. A familiar visual in manga and fiction during loud, distressing, or emotionally overwhelming moments — bracing oneself by physically blocking sound.