This is rather unbelievable for me. I have Wamp installed. PHP files were working fine until now. But suddenly there is some problem with it.

Whenever I try to open a file in browser, it pops up a window asking if I want to "Save File" or "Open With". If I select to "Open With" Firefox, it opens another firefox tab with the same pop up window.

Firefox is set as default browser for Wamp.

HELP ME OUT PLEASE.

Are you going in through the C drive and trying to open the files? Or straight through the wamp server?

If you haven't already, try using the "Localhost" link that is at the top of the Wamp menu. Then find your pages through that. It works a lot more smoothly than any other method.

opening a file from the file browser will not interpret the PHP and render it as HTML. to do that you MUST open the file in the (internet) browser, and further more it needs to be specified as a web page not a file location

eg http://localhost/path/to/file/filename.ext
as opposed to
c:/path/to/file/filename.ext

if you try to open the file in your internet browser as an absolute path to the file as opposed to a web page it will bot go through the web server. the server interprets the PHP and renders HTML data to the (internet) browser.