downloads script instead of loading page?

Hello, I have frustrated over this problem i have on my blog. And thought I’d ask. Whenever I am updating a post or editing anything on my blog It triggers me to download Post.php or admin.php or a script similar to what i am trying to edit, instead of loading the page. I have tested on other users and browsers it seems to only be doing it when logged in to admin and on firefox. Wondering if it is a security breech or just My browser. After it happens a couple times site stops working for 10-15 minutes on several browsers. Sometimes prompting you to download the script.
Any ideas or help is greatly appreciated. to let this continue is silly and going to be annoying.