In a recording obtained by TMZ ... Miley Cyrus' dad lays into Firerose over making him late for an event -- repeatedly calling her "an idiot" and a "dumbass."

The recording -- which sources close to Billy Ray say is heavily edited and doesn't accurately depict the situation -- features an argument between the now-separated couple, where BRC belittles Firerose several times.

Throughout the rant, Billy Ray accuses Firerose of not listening to him and walking all over him. He then goes on to rip into her for her health issues ... take a listen, as he seems to call out Firerore's BRCA1 gene mutation and pending double mastectomy surgery.

He adds ... "I don't think you're real smart. I changed my damn mind on that s**t. What you are is a selfish f***ing bitch."

From the sounds of it, Firerose tries to get Billy Ray to calm down, and calls him out for swearing at her. Billy Ray points the blame back at Firerose, however, saying the fight wouldn't be happening if she hadn't stirred the pot.

It's unclear from the recording alone what they're fighting about, but sources with direct knowledge tell TMZ ... the spat escalated over an invitation for Billy Ray to perform with Nicki Minaj at her Tennessee tour stop.

We're told both Billy Ray and Firerose thought it was all too last minute ... with the Disney Channel star later turning the blame on his estranged wife. However, sources close to Billy tell TMZ ... Billy's frustration stemmed from Firerose preventing him from attending the show ... with the Australian singer only suggesting they head to the concert after it was too late for BRC to hit the stage.

Billy Ray has since addressed the audio recording, defending his words as he was at his "wit's end."

He continued ... "As every day went by, I started realizing something was wrong. And that’s before I knew she was a fraud. I just knew something wasn’t right. That was before I knew she was David Hodges' ex-wife. That’s before I knew her parent’s last name. I had no idea she was arrested for felony residential burglary. I saw before my very own eyes, everything I thought I knew about her, was a lie. She was trying to take over my career, my life and usurp the Cyrus name for her own gain. See you in court."