Was there xrays done to determine the size & placement of the mass? Were you given options regarding surgery to remove the mass or does the vet think it is connected to or surrounding organs?
Has the vet mentioned kidney failure? The toxins not being removed from the body are going somewhere and the liver & the kidneys are the organs that deal with waste removal.
Has your cat had blood & urine work done recently?
I've never had liver issues in my cats, but my grrl has kidney failure which is being supported through a raw meat diet and kidney specific supplements from Standard Process.
Speak to your vet about their liver support specific supplements Standard Process Feline Hepatic Support
, Standard Process Feline Whole Body Support
is also a good overall support supplement.
You may also wish to speak to your vet about supplementing with Nu Cat Vitamins
and DMG supplement
Some info on Liver disease in cats