Rihanna's nasty legal battle with her ex-accountants is GETTING PERSONAL ... the firm is now trying to drag her mother into the case, but RiRi is telling them to back the hell off.
As TMZ first reported, Rihanna sued her ex-accounting firm, claiming it mismanaged her cash and took above-standard commission (22%) -- costing her millions. But the firm is adamant they did nothing wrong and says any loss in coin was Rihanna's own fault.
In the latest development ... the number crunchers fired off deposition requests to interview a slew of Rihanna's people involved in the case: assistants, tour managers, lawyers, real estate agents ... and her mother.
Specifically, the firm says RiRi's mom has direct knowledge of the situation -- she signed the contract since Rihanna was a minor at the time -- and they even offered to fly to Barbados (where mama lives) in order to have the chit chat.
But Rihanna shot down that idea, asking a judge to deny the request ... claiming the move is simply designed "to inconvenience, burden and harass" Rihanna through her family.
A judge didn't agree and overruled Rihanna's objection this week, allowing the firm to have a chit chat with Mama RiRi.