"maintained firm control" is correct and usable in written English.
You can use it when you want to describe a situation where someone has maintained strict oversight or authority over something. For example, "The CEO maintained firm control over the company's investments, ensuring that the company was never exposed to too much risk."
The PAP maintained firm control of Parliament through a succession of elections.
Mr. Kotik maintained firm control over the performance, which is to say that he brought a sense of drama to the on-the-spot decisions that Brown left to performers, and the ensemble responded with an involving vibrancy.
China's totalitarian government may have relaxed its control of the means of production, but it has maintained firm control of the means of reproduction, and continues to intrude into the most intimate aspects of an individual's life, stunting relationships, destroying traditional family life and spreading fear.
After an early opening shot from Erik Jendrisek of Slovakia, the Netherlands maintained firm control of the match until the 67th minute, when Miroslav Stoch cut inside from the wing and fired a powerful right-footed shot that Stekelenburg had to lunge to deflect.
Until the commencement of the French/ECOWAS (the Economic Community of West African States) campaign this month, the Islamist factions maintained firm control of a portion of Mali nearly equivalent to the size of France.
Even with her health problems, Ms. Alonso maintains firm control over the company.
He was the last Yuan ruler to maintain firm control over China, but he never exercised real power over Mongol territories in Russia and the Middle East.
But, as events rush toward a bloody resolution, Bernstein maintains firm control of his plot, and painstakingly re-creates the historical landscape in which an often reluctant Tausk undertakes his counter-revolutionary mission.
The country's cash-poor government has long seemed intent on following China's lead by maintaining firm control over the military and huge social-service bureaucracy, while introducing free-market measures to keep its economy afloat.
The United States, it said, "must maintain firm control of the West's nuclear strike force, if only because a feasible alternative, satisfying both military and political requirements, has not emerged".
"To the extent that State maintains firm control over Usaid, it can make it difficult for any agency to revitalize itself," said Connie Veillette, director of the program for rethinking foreign assistance at the Center for Global Development, an independent research group.
