It's rare for the Steelers to make major adjustments to the coaching staff during the season. Extremely rare.
Via Paul Hembekides of ESPN, the last time the Steelers fired a coordinator or head coach happened in 1941.
The coach at the time was Bert Bell. He previously owned the Eagles. After Art Rooney sold the Steelers to Alexis Thompson, Rooney bought half of Bell's interest the Eagles. They two franchises then decided to swap territories, with the Pirates becoming the Eagles and the Eagles becoming the Steelers.
Rooney became the G.M. and Bell became the coach. Bell resigned after two games in 1941.
The Steelers would make 10 more coaching changes before hiring Chuck Noll in 1969, who was followed by Bill Cowher in 1992 and Mike Tomlin in 2007. Bell would eventually become the NFL's Commissioner in 1946. He held the position until his death in 1959. The NFL has had three Commissioners since then: Pete Rozelle, Paul Tagliabue, and Roger Goodell.
So this really is a historic move by the Steelers. The question is whether it will improve the team's short-term future.