You must have some extra software or settings making that happen. Normal Windows and normal Firefox does not auto-correct web posts.
and see if that has the same problem. You might have a Firefox extension or plugin that's screwing with your posts.
“The egg hatched...” “...the egg hatched... and a hundred baby spiders came out...” (blade runner)
“Who are you?” “A friend. I'm here to prevent you from making a mistake.” “You have no idea what I'm doing here, friend.” “In specific terms, no, but I swore an oath to protect the world...” (continuum)
“It's a goal you won't understand until later. Your job is to make sure he doesn't achieve the goal.” (bsg)