"firmly assured" is correct and usable in written English.
You can use it to emphasize that something certain, reliable, or definite. For example: "The doctor firmly assured me that the treatment would be successful."
Firmly assured, though, of the truth of my evidence, I held up creditably against hostile cross-examination by the army's lawyer.
A L Kennedy This year No Great Mischief (Cape) made Alistair MacLeod's position as a master of prose even more firmly assured.
However, while the applicability and necessity of BAN have been firmly assured, the underlying technological platforms to practically realize these networks are still in the developmental stages, with many outstanding key problems to be addressed.
I had tried (and failed) to stop our two lead actresses from comparing their night-time enuresis issues at a dinner party in Beijing with two Red Army Generals who - I was firmly assured by the interpreter - understood not a word of English.
Place the damping materials on top of the glue, pressing firmly to assure adhesion.
Though this pressing question is left unanswered, we are firmly assured that the student in question is a "womanist" and has been oppressed accordingly.
He added, "I assured her that I firmly believed that what N.S.A. was doing was effective, appropriate, and lawful.
And we can rest assured that solid science firmly supports our efforts to offer more protection to the billions of animals who are abused in myriad ways.
Rest assured, most yogis are firmly planted in the real world and won't bore you to tears with a bunch of talk about the color of your aura.
Republican Governor John Kasich and his allies in Mitt Romney and Rick Perry may have waged war against organized labor, but rest assured that we stand firmly with you in this fight".
When the glue has been applied, hold the parts firmly together until proper adhesion is assured.
