Why is my doctor doing a biopsy of my lymph node?

Lymph node: He/she is concerned that it is either too big and/or firm and/or persisting.

Make a diagnosis: You have given very little information, so my answer must be very general. A biopsy of a lymph node is done almost always when the node in question is enlarged, and so the biopsy is to see why: is there cancer, whether lymphoma or metastatic cancer, or is there infection, or is there some other condition such as sarcoidosis? Only a biopsy can give a definitive answer.