Floyd Mayweather Jr. Lights $100 Bill ... ON FIRE

Floyd was partying with Lil Kim and Gucci Mane at the Velvet Room during the conflagration Sunday night -- and we're told, there were plenty more bills where that came from ... Floyd was rolling with STACKS of hundreds.

We should mention: it's illegal to burn money ... so if the Feds decide to pursue an investigation, Floyd could face up to six months behind bars, and, ironically, a $100 fine.

Fighting inflation, one bill at a time.