Billy Bob Thornton -- Hey Browns Fans ... STOP COMPLAINING!!!

They're still in the playoff race (technically), so Cleveland Browns fans need to QUIT THEIR BITCHIN' about the QB situation ... so says Billy Bob Thornton.

Billy Bob -- a huge Colts fan -- was leaving LAX yesterday after Indianapolis beat up on the Brownies ... when we asked if it was time to swap out Hoyer for Johnny Manziel.

Thornton essentially told us to RELAX when it comes to Johnny Football -- "He'll get in there one of these days."

But the best part ... "If I was a Browns fan and I had the record they have right now, I'd just be happy. you know? They're doing OK."