Which version of FF are you using? I've got 2 and i see your phpinfo() when i go to that page.
The typical cause of this is the server being configured incorrectly, but that usually results in every browser getting the download prompt (or one just displaying the php in plain-text, similar to how a .txt file would look).
Edit: Just remembered, FF has a "file types manager." Never messed with it much myself, but i know it changes how certain file types are handled. Maybe something could have gotten corrupted.