The new shadow defence secretary got funding for a legal research assistant from Leigh Day to support her work when she was shadow attorney general.
Leigh Day has been referred to a legal tribunal for its role in representing Iraqis allegedly mistreated by UK troops during the Iraq war.
The Labour MP declared each donation at the time they were received.
According to Parliament's register of interests, Leigh Day funded three secondments worth a total of Â£48,125 between 2012 and 2014.
Two other law firms also provided funding for a similar research role during that period.
During Prime Minister's Questions, David Cameron said there were "questions to answer" over the Leigh Day donations, while Labour MP John Mann has suggested Ms Thornberry should return what he described as "vested interest money".
Friends of Ms Thornberry said that soldiers pursuing claims against the Ministry of Defence were among Leigh Day's many clients, who also included people suffering from the effects of the drug thalidomide, for whom the prime minister called for extra support on Wednesday.
In an interview with British Forces Broadcasting Services marking her appointment, Ms Thornberry said Leigh Day was an "outstanding" firm and the allegations against it "had nothing to do with any of the outstanding youngsters who gave their time to help us out here".
Leigh Day, she said, had offered "some of their brightest and best to help make sure the opposition had proper legal advice".
Leigh Day was referred on Tuesday to the Solicitors Disciplinary Tribunal to answer complaints about its handling of legal challenges brought by Iraqi detainees against the Ministry of Defence.
It follows the findings of the 2014 Al-Sweady inquiry, which concluded that the most serious claims against British soldiers had been "deliberate lies, reckless speculation and ingrained hostility".
The firm has said the decision to refer it to the Solicitors Disciplinary Tribunal was "premature".
It has strongly denied allegations of wrongdoing, which, it said, it had not been given adequate time to respond to.
Friends of Emily Thornberry have said questions over donations she was given by a law firm are a "desperate smear".