It's possible the coding on that page was changed.

Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and

• Remove Cookies
  Warning ! ! This will log you out of sites you're logged in to.

Type about:preferences<Enter> in the address bar.

• Cookies; Select Privacy. Under History, select Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. Remove All of them.
• Cache; Select Advanced > Network. Across from Cached Web Content, Press Clear Now.

If there is still a problem, Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Then restart.

Separate Issue; Your System Details shows;

Installed Plug-ins

Adobe PDF Plug-In For Firefox and Netscape 10.1.2 Adobe PDF Plug-In For Firefox and Netscape 15.9.20069
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Adobe PDF Plug-In For Firefox and Netscape: Adobe Reader Version 2015.009.20069 Note: McAfee Security is offered in the download.

What security software do you have?

It is possible that security (Anti-Virus, firewall) is causing problems. Try to disable security software temporarily to see if that makes a difference.

Thanks. Problem is indeed caused by Microsoft Security Essentials. If I switch of "Real time protection (recommended)" the delay doesn't occur.

Is there a simple solution without switching to another anti-virus program?