with resolute determination
"we firmly believed it"
"you must stand firm"
In a firm or definite or strong manner.
"She held her ground firmly despite the opposing team's aggressive advances."
In plain English: Firmly means holding something tightly so it does not move easily.
"He held his breath firmly to keep from screaming."
The word firmly comes from Middle English fermely, which was formed by adding the suffix -ly to the adjective firm. It entered English with the meaning of "in a steady or solid manner," reflecting its original sense of being fixed and unyielding.