Plantar Fibroma: This is the most likely diagnosis due to the specific location (plantar foot) and the nature of the lesion (fibroma). Plantar fibromas are benign growths that arise from the plantar fascia and are known to cause discomfort or pain, especially with weight-bearing activities.

Plantar Wart: Although a wart is typically more superficial and may have a different appearance, it can sometimes be mistaken for a fibroma, especially if it is deep or has a similar texture. The presence of black dots (thrombosed capillaries) can help differentiate warts.
Morton's Neuroma: This condition involves a thickening of the tissue around the nerves leading to the toes, which can cause pain and discomfort in the ball of the foot. While not a fibroma, it can present with similar symptoms and is a common issue in the foot.
Ganglion Cyst: These are non-cancerous lumps that can develop on the underside of the foot and can sometimes be mistaken for fibromas due to their firm nature.

Kaposi's Sarcoma: Although rare, this malignant lesion can present as a firm, painful nodule on the foot and must be considered, especially in immunocompromised patients.
Melanoma: Any new or changing lesion on the foot, especially those that are pigmented, should raise concern for melanoma, a potentially deadly skin cancer if not caught early.
Squamous Cell Carcinoma: This type of skin cancer can appear as a firm, growing lesion and is more common in sun-exposed areas, but can also occur on the foot.

Desmoid Tumor (Aggressive Fibromatosis): A rare, locally aggressive tumor that can occur in the foot, though it is much less common than plantar fibroma.
Lipoma: While lipomas are common benign tumors, they are less frequently found on the plantar surface of the foot compared to other areas of the body.
Neurofibroma: Part of neurofibromatosis type 1, these can present as firm nodules but are less common on the plantar foot compared to other locations.