prompt to save/open php file instead of displaying WP site locally

whenever I try to go to localhost/wordpress in Firefox I get a prompt to save/open a php file (it does not give me the name of the file) instead of displaying the WP site properly. is there a firefox, MAMP, or wp setting I am missing somewhere? has anyone else run into this problem?

a screengrab of the prompt is here

any help would be appreciated.