The charity sector has come under increased scrutiny in Britain after the UK's worldwide development secretary Penny Mordaunt threatened to cut funding to any organisation that fails to ensure the highest standards are met on its staff members behaviour.
Oxfam later confirmed it sacked its country director in Haiti past year following allegations of mismanagement and inappropriate behaviour.
"The legal duty of all charity trustees is to act in the best interests of their charity and, in particular, to act with due care and diligence", it states.
Firth wrote: It is appalling to hear the news of the men that have abused the people they were there to help.
"Trustees should make sure that the policies are reviewed and kept up to date as organisations grow and change".
England and Wales's charity regulator is now investigating Oxfam's safeguarding procedures after it said the charity hadn't disclosed the full details of an internal inquiry into sex abuse allegations by its aid staff in Haiti.
"As such I am disassociating myself from Oxfam immediately", he said.
The charity said it was "not related to sexual misconduct". It would be a tragedy to see this relief work and advocacy stopped. For its part Oxfam must address this abuse diligently and transparently. The failure to deal with it adequately has further undermined that trust.
"Oxfam must do everything in its power to heal the damage to those who depend on both its work and the good faith and generosity of its supporters".
Firth added: What they were doing struck me as hard, but vitally important work.
The revelations came as Oxfam grapples with allegations that it was not transparent about a scandal involving some of its staff for using prostitutes in Haiti following a devastating 2010 quake.
She said she was horrified by the allegations. I could see that they gave scrupulous thought to how to help and empower the world's most vulnerable people.
International Development Secretary Penny Mordaunt vowed that sex abusers operating in the charity sector would be brought to justice "no matter where they are".
The scandal has already led to the resignation of Oxfam deputy chief executive Penny Lawrence.
Ms Mordaunt said: "No organisation is too big, or our work with them too complex, for me to hesitate to remove funding from them if we can not trust them to put the beneficiaries of aid first".
Oxfam officials also met the Charity Commission on Wednesday after the regulator launched a statutory inquiry.