Firefox isn't working allthough internet connection

I remember deleting a file 3 weeks ago, but forgot what it was (recycle bin already emptied). From that day on, neither Internet Explorer nor Firefox have been working.
I have already tried (several times) to install Firefox again but it didn't work.
Skype and my chat program are still working because i have got a working internet connection. It's just the internet browsers..
I don't know what it is and what i deleted but please help me!
Thank you!