"I don't firmly understand this".
yes e.g. he held her hand firmly.
Honestly
My word is my bond. The glue will bond your two pages together firmly.
The root word "mon" in "admonished" means to warn or scold someone firmly. In this sentence, the hall monitor cautioned or rebuked you for shouting.
he rested his broken leg firmly onto the hassock
Sealing usually means, to be encasing something in an envelope or another object, so a good sentence would be:When sealing an envelope, make sure that it is pressed down firmly.
He held her hand firmly.He was firmly set on his opinion.You are firmly in the wrong.
"The spade was dug firmly into the ground"
Can you use the word concluding in a sentence? Done.
You can use the word Truss in a sentence like this.
firmly
Just use it! Or do you mean, can you use the word beheld in a sentence.