Snoop Dogg -- BLASTS STEELERS COACH ... 'Fire That Motherf**ker!'

Snoop Dogg thinks the Pittsburgh Steelers' offensive coordinator SUCKS -- and last night he blasted the guy in an epic Instagram rant ... saying, "Fire that motherf**ker!"

FYI -- Snoop's a huge Steelers fan and has been for years ... and clearly he wasn't impressed with the team's pre-season loss to the Eagles last night.

But Snoop isn't one to sit on the sidelines, so he made a video -- begging head coach Mike Tomlin to fire the hell out of O.C. Todd Haley ... saying, " We ain't won a playoff game since we had this motherf**ker, man. S**t."