The phrase "in firmest" can be substituted with synonyms such as resolute, steadfast, unyielding, determined, and unwavering. These words all convey a sense of unwavering conviction or determination, indicating a strong commitment to a particular belief or course of action. Other synonyms for "in firmest" could include inflexible, tenacious, persistent, and uncompromising, which all suggest an unwavering commitment in the face of opposition or difficulty. Regardless of which synonym is used, it is clear that the speaker or subject is unshakeable in their resolve and dedicated to achieving their goals, no matter what challenges or obstacles arise.