Ganglion cyst: This is a common condition where a lump forms under the skin, often on the wrist or foot. It's usually firm, tender, and not movable, which matches the symptoms described. Ganglion cysts can occur on the dorsal part of the foot and are known for their firmness and tenderness.

Bursitis: Inflammation of a bursa (a fluid-filled sac) near the joint can cause a hard, tender mass. On the foot, this could be due to repetitive pressure or friction, leading to bursitis on the dorsal aspect.
Bone spur or osteophyte: A bony growth that can develop on the edges of joints due to aging, injury, or conditions like arthritis. On the dorsal part of the foot, it could feel like a hard mass and be tender to the touch.
Tendinosis or tendonitis: Inflammation or degeneration of the tendons, which could present as a firm, tender mass, especially if there's a nodule or thickening of the tendon on the dorsal foot.

Malignant tumor (e.g., sarcoma): Although rare, a hard, fixed mass could be a sign of a malignant tumor. Early detection is crucial for treatment, making this a "do not miss" diagnosis.
Infection (e.g., abscess or osteomyelitis): An infection in the bone or soft tissue could present as a hard, tender mass. Identifying and treating infections promptly is critical to prevent serious complications.
Gouty tophus: A deposit of uric acid crystals in the soft tissue, which can form a hard, tender nodule. While more commonly associated with the joints, tophi can occur in various locations, including the foot.

Foreign body granuloma: A reaction to a foreign object (like a splinter) that has entered the skin, leading to the formation of a firm, tender mass.
Neurofibroma: A benign tumor of the nerve tissue, which can present as a firm mass. Though rare, it's an important consideration, especially in patients with neurofibromatosis.
Xanthoma: A rare condition where fatty growths develop under the skin, which could potentially feel like hard masses, though they are typically less tender than other conditions listed.