This problem is caused by AVG Safe Search and AVG Security Toolbar which you have installed.

Uninstall both by clicking the orange Firefox button, go to Add-ons, then Extensions. You'll find that pages will load properly afterwards.

Firefox has its own built-in security which will warn you if a site is untrustworthy, so you don't need to be concerned about system security.

See also: http://kb.mozillazine.org/Problematic_extensions

If this suggestion resolves the problem for you, please click the Solved it button next to this post after you log in into the forum. This will help others searching for a solution to the same problem.

Thanks.

Firefox never finishes loading certain websites

https://support.mozilla.com/en-US/kb/Firefox%20never%20finishes%20loading%20certain%20websites

Check and tell if its working.