Ascites: This condition, characterized by an accumulation of fluid in the peritoneal cavity, is a common cause of abdominal swelling and can be associated with pain and firmness due to the stretching of the abdominal wall and potential underlying liver disease or other causes.

Irritable Bowel Syndrome (IBS): Although IBS is more commonly associated with abdominal pain and changes in bowel habits, some patients may experience bloating that could be perceived as swelling.
Constipation: Severe constipation can lead to abdominal distension and pain, and if the constipation is due to a mass or obstruction, it could also cause firmness.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause abdominal pain, swelling due to inflammation or obstruction, and firmness due to the chronic nature of the disease.

Abdominal Aortic Aneurysm: Although it may not directly cause swelling, an expanding aneurysm can cause abdominal pain and a palpable, firm mass. Missing this diagnosis can be fatal.
Ovarian Torsion: This is a gynecological emergency that can cause severe abdominal pain and may lead to swelling and firmness if there is associated hemorrhage or necrosis.
Appendicitis: While typically causing localized pain, a ruptured appendix can lead to peritonitis, which may cause generalized abdominal pain, swelling, and firmness.
Intestinal Obstruction: This condition can cause severe abdominal pain, vomiting, and swelling due to the accumulation of fluid and gas proximal to the obstruction.

Abdominal Lymphoma: Can cause abdominal swelling due to the accumulation of lymphomatous cells in the abdomen and may cause pain and firmness due to the involvement of abdominal organs.
Tuberculous Peritonitis: A form of tuberculosis that affects the peritoneum, leading to ascites, abdominal pain, and swelling.
Pseudomyxoma Peritonei: A clinical syndrome characterized by recurrent mucinous ascites causing abdominal swelling, often associated with a primary mucinous tumor of the appendix.