"firmly secure" is correct and usable in written English.
You can use it when you want to describe something that is strongly or firmly in place. For example, "He firmly secured the rope around the post."
After three weeks in my new role, I believe we are on the cusp of a new era, which if grasped will firmly secure Britain's continued role as a leader both in the arts, and the creative industries.
Nothing I've ever achieved has been enough for me to firmly secure my father's pride, affection or acceptance.
Some were celebrated, some were criticised for being wildly offensive – but more than anything, they helped him firmly secure his spot in skateboarding history.
Anwar Sadat was convinced that a regiment reinforced with aircraft could firmly secure Al-Sallal and his free officer movement, but within three months of sending troops to Yemen, Nasser realized that this would require a larger commitment than anticipated.
The fixation plates were designed by thickening the outer ideal contour surface of the mandible to firmly secure the HA scaffolds to the bone and ensure primary stability during mastication.
Firmly secure the lid over the blender.
The ultrasound probe was firmly secured on the muscle belly using a lightweight foam fixation.
I checked once again to make sure my false eyelashes were firmly secured.
Seven minutes later, a series of clamps firmly secured the vehicles.
"Les Miz" will nevertheless bow out having firmly secured its place in Broadway history.
With its two nearest enemies - Saddam Hussein and the Taliban - gone, Iran has firmly secured its interests in both Iraq and Afghanistan.
