Why?

"However, studies have shown that famotidine is not effective in reducing mortality or improving recovery in COVID-19 patients.[38]" Borrell B (26 April 2020). "New York clinical trial quietly tests heartburn remedy against coronavirus". Science Magazine.

A lot of Long Covid seems to be related to mitochondrial damage. You'd probably be better off getting some creatine (which is commonly available because weightlifters use it) and seeing if that helps. Creatine is at least well documented to improve mitochondrial funciton.