Jon started the latest round this week ... taunting Cormier on social media for failing to accept the "truth" that PEDs had nothing to do with JJ's two UFC victories over him.

Jones says the U.S. Anti-Doping Agency has backed his claims that he didn't intentionally ingest any prohibited substances ... and insists Cormier simply can't come to grips with the fact he's the superior fighter.

Enter Cormier ... who fired back on social media Friday morning.

"So here we go again Jonny boy. First off, if you did nothing wrong and were 'innocent' then you’d have never been suspended," Cormier said.

"And on top of all the cheating you’ve done around UFC 182, UFC 200 and UFC 214 that’s not even the straw that broke the camel’s back. You are actually a SNITCH!"

Cormier is referring to reports that Jones cut a deal with USADA for reduced 15-month suspension after his positive PED test -- and in exchange, Jones provided "substantial assistance" to the agency.

Many believe (including Cormier) the aforementioned "assistance" was info about other fighters -- aka "snitching."

Back to Cormier's attack ...

"You think you’re the only one that’s been offered this deal? No, you’re just the only scumbag low enough to take it!"

"Which one of your teammates did you tell on to get this joke of a punishment from USADA! I thought there was nothing lower than a cheater but it’s not surprising that you show me a whole new level."

"You’re a cheater and a SNITCH! Get off my page and go do some coke, or steroids or whatever you losers do in your spare time. #doublechamp"