Does the page reload, but no results appear? Check whether you have JavaScript enabled. In particular, if you use the NoScript extension, make an exception for the site.

The page does not seem to reload. I have JavaScript enabled and I have not installed the NoScript extension. I installed the latest version of Java but nothing seems to happen. Does it work for you? Please try to search for something in http://preview.ncbi.nlm.nih.gov/pubmed. Thanks in advance!

Yes, I searched for dark chocolate and it returned this page:

http://preview.ncbi.nlm.nih.gov/pubmed?term=dark%20chocolate

When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

1. Clear Firefox's Cache

orange Firefox button or Tools menu > Options > Advanced

On the Network mini-tab, Offline Storage: "Clear Now"

2. If needed, delete the site's cookies here

While viewing that home page, right-click and choose View Page Info > Security > "View Cookies"

Carefully remove the site's cookies individually, do NOT click remove "all" unless you really want to remove all your cookies (there is no undo for this dialog).

Does that help?