Strength and Stability:
* Securely
* Steadily
* Tightly
* Strongly
* Resolutely
* Solidly
* Immovably
Determination and Conviction:
* Resolutely
* Determinedly
* Decisively
* Unwavering
* Insistently
* Conclusively
Other synonyms:
* Tightly (for physical objects)
* Securely (for physical objects)
* Deeply (for feelings or beliefs)
* Certainly (for statements)
* Absolutely (for statements)
Examples:
* "He held the steering wheel firmly." (Securely, tightly)
* "She stood her ground firmly." (Resolutely, steadfastly)
* "I believe firmly in the importance of education." (Strongly, deeply)
* "The evidence firmly points to his guilt." (Conclusively, decisively)
Choosing the best synonym depends on the specific meaning you want to convey.