Installation issues on local host (MAMP)

Hi
I’ve installed MAMP fine and everything is running fine (well it told me it is), I copied the full WordPress 3.0.1 folder to the relevant location and changed the config file as specified in the instructions. When I go to the wp-admin/install.php though on Firefox I get a pop up box asking what I want to do with the file – either open in an application I select or save file. Selecting open in Firefox brings up a blank page and the same dialogue box.

What am I missing? I’ve installed wordpress a number of times on a remote server but would like the convenience of hosting a version locally for design/testing etc.