It is possible that the page was zoomed out, Firefox remembers zoom setting for each site. For how to reset the zoom see the page zoom article.

Thanks a lot TonyE. It worked like magic. I was posting an article on my wordpress powered blog and must have mistakenly hit some buttons. It is now over 2 months since I have been trying all sorts of methods to return it to normal but with no success.

Initially, I didn't mind thinking maybe it's only me that is seen the shrunk pages but then I realized that my visitors may be seeing the same thing because I noticed that my clickthrough rate fell drastically then I googled for solutions.

You are amazing and thank you.