Gilbert Arenas -- Back On Probation ... After Illegal Fireworks Bust

Gilbert Arenas is back on probation ... stemming from an illegal fireworks bust from 2013 ... TMZ Sports has learned.

The former NBA superstar -- who was famously involved in a NBA locker room gun showdown back in 2010 -- was arrested in L.A. with 100 POUNDS of illegal explosives in June 2013.

The timing couldn't have been better for Arenas ... who had completed probation from the gun incident roughly one year before the fireworks arrest. Talk about a close call.

As for the fireworks bust ... Arenas was charged with misdemeanor fireworks possession and was facing jail time -- but ultimately cut a deal with prosecutors to get probation instead.

According to official docs obtained by TMZ Sports, Arenas pled no contest ... and in exchange was hit with $41,240 in fines, penalties and other fees. Plus, he was ordered to complete 10 days of community service pertaining to "fire prevention."

Court docs show Arenas paid the fines and completed the community service -- and will remain on probation until January 2016.

Translation -- probably best to leave the fireworks to the pros this year.